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How can I improve performance for multiple images drawn in webgl?

I am programming a simple webgl application, which draws multiple images (textures) over each other. Depending on the scroll position, the scale and opacity of the images is changed, to create a 3D multi-layer parallax effect. You can see the effect here:
http://gsstest.gluecksschmiede.io/ct2/
I am currently working on improving the performance, because the effect does not perform well on older devices (low fps). I lack the in-depth knowledge in webgl (and webgl debugging) to see what is the reason for the bad performance, so I need help. This question only is concerned with desktop devices.
I've tried / I am currently:
always working with the same program and shader pair
the images are in 2000x1067 and already compressed. I need png because of the transparency. I could compress them a little more, but not much. The resolution has to be that way.
already using requestAnimationFrame and non-blocking scroll listeners
The webgl functions I am using to draw the image can be read in this file:
http://gsstest.gluecksschmiede.io/ct2/js/setup.js
Shader code can be found here (just right click -> show sourcecode):
http://gsstest.gluecksschmiede.io/ct2/
Basically, I've used this tutorial/code and did just a few changes:
https://webglfundamentals.org/webgl/lessons/webgl-2d-drawimage.html
I'm then using this setup code to draw the images depending on current scroll position as seen in this file (see the "update" method):
http://gsstest.gluecksschmiede.io/ct2/js/para.js
In my application, about 15 images of 2000x1067 size are drawn on to each other for every frame. I expected this to perform way better than it actually is. I don't know what is causing the bottleneck.
How you can help me:
Provide hints or ideas what code / image compression / whatever changes could improve rendering performance
Provide help on how to debug the performance. Is there a more clever why then just printing out times with console.log and performance.now?
Provide ideas on how I could gracefully degrade or provide a fallback that performance better on older devices.

This is just a guess but ...
drawing 15 fullscreen images is going to be slow on many systems. It's just too many pixels. It's not the size of the images it's the size they are drawn. Like on my MacBook Air the resolution of the screen is 2560x1600
You're drawing 15 images. Those images are drawn into a canvas. That canvas is then drawn into the browser's window and the browser's window is then drawn on the desktop. So that's at least 17 draws or
2560 * 1600 * 17 = 70meg pixels
To get a smooth framerate we generally want to run at 60 frames a second. 60 frames a second means
60 frames a second * 70 meg pixels = 4.2gig pixels a second.
My GPU is rated for 8gig pixels a second so it looks like we might get 60fps here
Let's compare to a 2015 Macbook Air with a Intel HD Graphics 6000. Its screen resolution is 1440x900 which if we calculate things out comes to 1.3gig pixels at 60 frames a second. It's GPU is rated for 1.2gig pixels a second so we're not going to hit 60fps on a 2015 Macbook Air
Note that like everything, the specified max fillrate for a GPU is one of those theoretical max things, you'll probably never see it hit the top rate because of other overheads. In other words, if you look up the fillrate of a GPU multiply by 85% or something (just a guess) to get the fillrate you're more likely to see in reality.
You can test this easily, just make the browser window smaller. If you make the browser window 1/4 the size of the screen and it runs smooth then your issue was fillrate (assuming you are resizing the canvas's drawing buffer to match its display size). This is because once you do that less pixels are being draw (75% less) but all the other work stays the same (all the javascript, webgl, etc)
Assuming that shows your issue is fillrate then things you can do
Don't draw all 15 layers.
If some layers fade out to 100% transparent then don't draw those layers. If you can design the site so that only 4 to 7 layers are ever visible at once you'll go a long way to staying under your fillrate limit
Don't draw transparent areas
You said 15 layers but it appears some of those layers are mostly transparent. You could break those apart into say 9+ pieces (like a picture frame) and not draw the middle piece. Whether it's 9 pieces or 50 pieces it's probably better than 80% of the pixels being 100% transparent.
Many game engines if you give them an image they'll auto generate a mesh that only uses the parts of the texture that are > 0% opaque. For example I made this frame in photoshop
Then loading it into unity you can see Unity made a mesh that covers only the non 100% transparent parts
This is something you'd do offline either by writing a tool or doing it by hand or using some 3D mesh editor like blender to generate meshes that fit your images so you're not wasting time trying to render pixels that are 100% transparent.
Try discarding transparent pixels
This you'd have to test. In your fragment shader you can put something like
if (color.a <= alphaThreshold) {
discard; // don't draw this pixel
}
Where alphaThreashold is 0.0 or greater. Whether this saves time might depend on the GPU since using discarding is slower than not. The reason is if you don't use discard then the GPU can do certain checks early. In your case though I think it might be a win. Note that option #2 above, using a mesh for each plane that only covers the non-transparent parts is by far better than this option.
Pass more textures to a single shader
This one is overly complicated but you could make a drawMultiImages function that takes multiple textures and multiple texture matrices and draws N textures at once. They'd all have the same destination rectangle but by adjusting the source rectangle for each texture you'd get the same effect.
N would probably be 8 or less since there's a limit on the number of textures you can in one draw call depending on the GPU. 8 is the minimum limit IIRC meaning some GPUs will support more than 8 but if you want things to run everywhere you need to handle the minimum case.
GPUs like most processors can read faster than they can write so reading multiple textures and mixing them in the shader would be faster than doing each texture individually.
Finally it's not clear why you're using WebGL for this example.
Option 4 would be fastest but I wouldn't recommend it. Seems like too much work to me for such a simple effect. Still, I just want to point out that at least at a glance you could just use N <div>s and set their css transform and opacity and get the same effect. You'd still have the same issues, 15 full screen layers is too many and you should hide <div>s who's opacity is 0% (the browser might do that for you but best not to assume). You could also use the 2D canvas API and you should see similar perf. Of course if you're doing some kind of special effect (didn't look at the code) then feel free to use WebGL, just at a glance it wasn't clear.

A couple of things:
Performance profiling shows that your problem isn't webgl but memory leaking.
Image compression is worthless in webgl as webgl doesn't care about png, jpg or webp. Textures are always just arrays of bytes on the gpu, so each of your layers is 2000*1067*4 bytes = 8.536 megabytes.
Never construct data in your animation loop, you do, learn how to use the math library you're using.

Detect if browser uses GPU accelerated graphics for render

So I'm building large size site, which uses css3 transitions for animations (I'm using jaquery.transit to manipulate element transitions and their css styles). And I stumbled upon 2 problems:
FF (latest update) doesn't use GPU for translate3d() layer rendering, maybe I'm wrong and mozilla doesn't use GPU accelerated graphics at all. I really don't understand that completely yet.
Even if I trick for example Chrome in using GPU for translate3d() and translateZ() layer rendering, on computers with bad GPU or with no GPU those graphics are so terrible you sometimes can't even see middle of transition just start and end.
Questions:
What do I do to improve FPS for transitioning elements, e.g. I have 3200x3200 div rotating and scaling and translating in x,y axis at same time, with approx. 5-20 elements displayed on that div's surface?
Maybe there is a way I can detect if browser has enough GPU support to know if I need to redirect to simpler version of site or not?

Because WebGL uses GPU, the amazing Modernizr project permits to check that for webGL supported browsers: http://modernizr.com/news/
Check Modernizr.webgl under http://modernizr.com/docs/

The main issue at the time was that all of the PNG's on the screen had to recalculated and recompiled in the browser.
There were several things I had to do to to maximize performance:
Always have predefined width and height attributes on images. What this does is let's the browser know what size the picture should be and when used together with scale() it won't recalculate and recompile those images. These things were very expensive. So basically if nothing else than scale() modified image size, everything was perfect and animations were awesome.
Wherever possible avoid using visibility property, it literally acts like opacity: 0 keeping the element in the layout making layout recalculation much longer. Always where possible use display: none, this will completely eliminate element from the layout calculations. This was a major pitfall, because I had to re-think the UI to exclude visibility and I had to minimize used DOM node count.
Overall it was a huge adventure and big experience, hope this question/answer helps someone.

Drawing lots of hexagons and distorting them

I'm starting a web game, and I would like to know the best way to do it.
The game is a field of hexagons seen from the above, that can be rotated/inclined.
What's the best way to do it? What I was going to do was use a canvas and use 2d transforms to simulate 3d rotations, the problem is that:
The game must work on smartphones
For every rotation i must redraw all the canvas, and there could be 200 hexagons on the screen to redraw many times, so i think canvas are too expensive in terms of resources...

There's two ways I can think of:
Using canvas only. This means quite wide browser support, will have few quirks and generally "just work". Potential problems: Performance on low-end machines like you mentioned. But is rotating the game critical to gameplay? If not, you could consider turning that off on slow smartphones. Users will most likely not miss that feature if it's not important to gameplay.
A combination of canvas and CSS3 transforms (rotating cube example). Could give you better performance than pure canvas solution. Potential problems: requires "mixing" of techniques, which always means a risc of running into unexpected problems or quirks.

You should look at www.kineticjs.com
Canvas is the way to go for drawing 100's of shapes because it doesn't write to the DOM for every path like SVG does.
It has several examples where shapes are programmatically drawn in the 1000s. I recently used it for a similar animation here:
http://movable.pagodabox.com

What's the fastest way to draw to an HTML 5 canvas?

I'm investigating the possibility of producing a game using only HTML's canvas as the display media. To take an example task I need to do, I need to construct the game environment from a number of isometric tiles. Of course, working in 2D means they by necessity come in rectangular packages so there's a large overlap between tiles.
I'm old enough that the natural solution to this problem is to call BitBltMasked. Oh wait, no, an HTML canvas doesn't have something as simple and as pleasing as BitBlt. It seems that the only way to dump pixel data in to a canvas is either with drawImage() which has no useful drawing modes that ignore the alpha channel or to use ImageData objects that have the image data in an array.. to which every. access. is. bounds. checked. and. therefore. dog. slow.
OK, that's more of a rant than a question (things the W3C like tend to provoke that from me), but what I really want to know is how to draw fast to a canvas? I'm finding it very difficult to ditch the feeling that doing 100s of drawImages() a second where every draw respects the alpha channel is inherently sinful and likely to make my application perform like arse in many browsers. On the other hand, the only way to implement BitBlt proper relies heavily on a browser using a hotspot-like execution technique to make it run fast.
Is there any way to draw fast across every possible implementation, or do I just have to forget about performance?

This is a really interesting problem, and there's a few interesting things you can do to solve it.
First, you should know that drawImage can accept a Canvas, not just an image. The "sub-Canvas"es don't even need to be in the DOM. This means that you can do some compositing on one canvas, then draw it to another. This opens a whole world of optimization opportunities, especially in the context of isometric tiles.
Let's say you have an area that's 50 tiles long by 50 tiles wide (I'll say meters for the sake of my own sanity). You might divide the area into 10x10m chunks. Each chunk is represented by its own Canvas. To draw the full scene, you'd simply draw each of the chunks' Canvas objects to the main canvas that's shown to the user. If only four chunks (a 20x20m area), you would only perform four drawImage operations.
Of course, each of those individual chunks will need to render its own Canvas. On game ticks where nothing happens in the chunk, you simply don't do anything: the Canvas will remain unchanged and will be drawn as you'd expect. When something does change, you can do one of a few things depending on your game:
If your tiles extend into the third dimension (i.e.: you have a Z-axis), you can draw each "layer" of the chunk into its own Canvas and only update the layers that need to be updated. For example, if each chunk contains ten layers of depth, you'd have ten Canvas objects. If something on layer 6 was updated, you would only need to re-paint layer 6's Canvas (probably one drawImage per square meter, which would be 100), then perform one drawImage operation per layer in the chunk (ten) to re-draw the chunk's Canvas. Decreasing or increasing the chunk size may increase or decrease performance depending on the number of update you make to the environment in your game. Further optimizations can be made to eliminate drawImage calls for obscured tiles and the like.
If you don't have a third dimension, you can simply perform one drawImage per square meter of a chunk. If two chunks are updated, that's only 200 drawImage calls per tick (plus one call per chunk visible on the screen). If your game involves very few updates, decreasing the chunk size will decrease the number of calls even further.
You can perform updates to the chunks in their own game loop. If you're using requestAnimationFrame (as you should be), you only need to paint the chunk Canvas objects to the screen. Independently, you can perform game logic in a setTimeout loop or the like. Then, each chunk could be updated in its own tick between frames without affecting performance. This can also be done in a web worker using getImageData and putImageData to send the rendered chunk back to the main thread whenever it needs to be updated, though making this work seamlessly will take a good deal of effort.
The other option that you have is to use a library like pixi.js to render the scene using WebGL. Even for 2D, it will increase performance by decreasing the amount of work that the CPU needs to do and shifting that over to the GPU. I'd highly recommend checking it out.

I know that GameJS has blit operations, and I certainly assume any other html5 game libraries do as well (gameQuery, LimeJS, etc etc). I don't know if these packages have addressed the specific array-bounds-checking concern that you had, but in practice their samples seem to work plenty fast on all platforms.
You should not make assumptions about what speedups make sense. For example, the GameJS developer reports that he was going to implement dirty rectangle tracking but it turned out that modern browsers do this automatically---link.
For this reason and others, I suggest to get something working before thinking about the speed. Also, make use of drawing libraries, as the authors have presumably spent some time optimizing performance.
I have no personal knowledge about this, but you can look into the appMobi "direct canvas" HTML element which is allegedly a much faster version of normal canvas, link. I'm confused about whether this works in all browsers or just webkit browsers or just appMobi's own special browser.
Again, you should not make assumptions about what speedups make sense without a very deep knowledge of web browser internal processes. That webpage about "direct canvas" mentions a bunch of things that slow down canvas-drawing: "Reflowing text, mapping hot spots, creating indexes for reference links, on and on." Alpha-blending and array-bounds-checking are not mentioned as prominent causes of slowness!

Unfortunately, there's no way around the alpha composition overhead. Clipping may be one solution, but I doubt there would be much, if any, performance gain. Not to mention how complicated such a route would be to implement on irregular shapes.
When you have to draw the entire display, you're going to have to deal with the performance hit. Although afterwards, you have a whole screen's worth of pre-calculated alpha imagery and you can draw this image data at an offset in one drawImage call. Then, you would only have to individually draw the new tiles that are scrolled into view.
But still, the browser is having to redraw each pixel at a different location in the canvas. Which is quite expensive. It would be nice if there was a method for just scrolling pixels, but no luck there either.
One idea that comes to mind is that you could implement multiple canvases, translating each individual canvas instead of redrawing the pixels. This would allow the browser to decide how to redraw those pixels, in a more native way, at least in theory anyway. Then you could render the newly visible tiles on a new, or used/cached, canvas element. Positioning it to match up with the last screen render.
But that's just my two blits... I mean bits... duh, I mean cents :]

Break the scene to several canvases or keep redrawing one?

Intent on creating a canvas-based game in javascript I stand before a choice:
Should I performance-wise keep all the stuff happening in the screen in one canvas (all the moving characters, sprites) and redraw it at constant rate of, say, 60 FPS or should I break the scene into several smaller canvases thus removing the need of redundant redrawing of the scene? I could even create separate canvas elements for the characters' limbs and then do most of the animation by simply manipulating the CSS of the given canvas element (rotation, positioning, opacity).
To me the latter sounds way more plausible and easier to implement, but is it also faster? Also, shouldn't I perhaps use SVG, keep the characters and sprites as elements inside of it and manipulate their XML and CSS properties directly?
So what do you think is the most fitting solution to a scene with severals sprites and characters:
One canvas object redrawn manually (and wastefully) at FPS rate
Several canvas elements, redrawn manually in a more reasonable fashion
Structured vector graphics document like SVG / VML manipulated via DOM
I am mainly concerned about the performance differences, but the legibility of the logical code behind is also of interest (I, having already worked with canvas before, am for example fairly sure that the redrawing function for the entire canvas would be one hard-to-maintain beast of a script).

DOM manipulations are slow in comparison to GPU-accelerated canvas operations, so I would stay away from SVG and VML.
As far as structuring your canvas code goes, it certainly doesn't make sense (especially for performance reasons) to clear and re-draw the entire canvas because the player moved or performed an action. Based on your description here, I'm guessing that your game will be 2D. These types of games lend themselves extremely well to layering unless you're doing something rather complex like Paper Mario. You should be looking at the issue from an object-oriented viewpoint and encapsulating your drawing procedures and objects together as appropriate.
For instance, create a player object that maintains a small canvas representing the character. All the logic needed to maintain the character's state is kept within the object and any changes made to it need not worry about other components of the game's visual representation. Likewise, do the same for the background, user interface, and anything else you can abstract into a layer (within reason). For example, if you're doing a parallax game, you might have a foreground, background, character, and user interface layer.
Ultimately you will need to maintain the state of the different components in your game individually. Player animations, background clouds moving, trees swaying, etc. will all be managed by appropriate objects. Since you will already have that type of code structure setup, it makes sense to just maintain major components on separate canvas elements and composite them together as needed for better performance. If the character moves in the bottom left corner of a scene with a static background, you don't need to re-draw the top right corner (or 95% of the scene, for that matter). If you're considering full-screen capabilities, this is definitely a performance concern.

There's a rule in programming, that you should never try and optimize something before you have a speed problem with it. You'll spend more time trying to figure out how to code something in the best way than to actually code, and will never finish anything.
Draw them all on your canvas at a fixed rate. That's how it's done. If you start creating a canvas for each limb and element and manipulate them using CSS, you're wasting the potential of canvas. Might as well just use images. That's how they did it before canvas. That's the problem canvas was made to solve.
If you ever encounter speed issues, then you can start hammering at them. Check out these slides for some tips (related video). This guy's blog also has some handy tips on canvas performance.