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Safari much slower than other browsers

I am trying to build a Wordpress website using a theme I purchased, unfortunately the maker of the theme doesn't seem to respond to any support requests.
After creating a few pages I noticed that performance is much worse on Safari compared to any other browser. I tested this on a few computers and few versions of macOS.
Example page is https://sochacki.info/proj/nepal-manaslu-circuit/
I realise that photos on the website are quite big (4000px each), but that is exactly what I want. I am happy with how quickly the pages load and how the galleries work, in Chrome, Firefox or Opera. When you click on a picture it loads PhotoSwipe JavaScript gallery so you can see them in full size, and then you can also click in top right corner to go full screen as well, or to leave the PhotoSwipe viewer. Switching between photos, going full screen or leaving PhotoSwipe are instant.
However when I load the same page in Safari, any action after clicking on a photo is slow. Switching between photos, going full screen, leaving full screen. It all takes a second or a few seconds. I can also see in Activity Monitor that CPU is used way more in Safari.
I tried a couple of things:
the theme I am using uses a custom plugin made by the same author to create these galleries, place photos and it includes PhotoSwipe files inside. Those files were one version behind (4.1.2 instead of latest 4.1.3) so I updated them, but the performance issues are the same.
I installed some other gallery plugins which use PhotoSwipe to display photos, and they did not have the performance issues, so I am not blaming it on PhotoSwipe, probably some other part of the plugin is causing the issues.
I have no real experience with JS or web development, are there any tools that could help me find what is causing the slow performance?

I checked the website speed - as I can see the Theme-Loading-time is okay. But as you already mentioned the Photo-sizes are really unfortunate as we can see in google page insights (https://developers.google.com/speed/pagespeed/insights/?hl=de&url=https%3A%2F%2Fsochacki.info%2Fproj%2Fnepal-manaslu-circuit%2F)
Try to resize the Photo-sizes in normal view (not the HeightxWidth > the kB)

Site not loading properly on mobile

I am having issues trying to load my site, http://www.internhacks.io/, on any mobile browser.
The project can be found here on Github.
I have tested the site on mobile using Chrome dev tools, and everything loads fine/acts responsively.
However, when testing the site on my actual phone, sometimes the site does not load at all, sometimes only partially.
Note: the apply button is not meant to do anything yet
I think it may have to do with having a large image as the background. Should I be serving a smaller version if detecting the window is smaller? The img height is set to 100vh.
If anyone knows what might be causing this, or knows of a better way to debug the site on mobile than in Chrome dev tools, please let me know!

I use Opera mini to test mobile devices (aka android) with various screen resolutions.
"height is set to 100vh" huh? never saw that one before. To fit graphics within space available, avoid fixed sizes (aka 100px) and I use relative width:xx%
YES, it's wasteful to send large graphics to a mobile device and from the server, you need to send some xxx-low-res.img instead
it's helpful to load JS scripts into a section to allow all objects to be loaded first.

I think your main issues is that the big images should use smaller, light-weight pictures instead since it'll speed up the resize process when rendering on the phone. Other than that, you want to stick away from, as #jobeard mentioned, from using fixed sizes like the 100vh and use a relative width such as 10%.

Down-Sized Images are Slow in Chrome

I'm working on a script and have an issue with Chrome.
Here's what I'm doing:
1. Loading large images into a DIV.
2. Scaling the images to fit the size of the DIV.
3. When the browser is resized, the image is scaled up and down with the browser.
Everything is working 100% in all browsers except Chrome.
I've checked the profiler in Chrome and don't see anything unusual.
These are large images, however are working just fine (even live) in all other browsers.
I've read a ton of places that have noted this issue when using down-sized images in Chrome...but not solution.
Nothing special going on, just using a
var img=new Image();
$(img).load(function(){ .... {);
Does anyone know of a workaround or solution that will work in Chrome?
Thanks!

So it appears that Chrome just sucks when it comes to handling large images and even images that aren't large, but just scaled down. I've searched around endlessly only to find similar questions without resolutions.
Still not sure why all other browsers (even IE 7 and 8) can handle large images (tested with 7mb scaled-down PNGs), but Chrome can't even manage to do a 700kb scaled-down JPG without lagging.
So, I'm answering this question with: in this instance, Chrome sucks.

I had exactly the same problem, but recently solved it. Huge amounth of performance takes up chrome's anti-aliasing, because it recalculates the entire image whenever you resize it.
So to solve it you just add this line to the css if your image:
#myImage
{
image-rendering: -webkit-optimize-contrast;
}
You can turn it off by javascript when the image is still.
More about anti-aliasing: Disable antialising when scaling images

How are you resizing the images? Using height/width or CSS transforms? The latter might result in better performance.
Some info:
Improving the Performance of your HTML5 App

When you're profiling issues with image resizes and decodes in Chrome, it's best to use the Developer Tools Timeline for this as it can give you relatively accurate stats on exactly what took a long time to be decoded.
Downsizing (or re-sizing) involves Chrome having to both decode the image (JPEG/PNG/GIF) you're sending down the line and then do extra work to resize that image into the container (div) that you would like to display it in. Where possible, the advice from the Chrome team is to prescale your images to the correct width/height needed.
Now you might be wondering: well, surely this issue is just down to desktop Chrome sucking, right? It's not as clear-cut as that. Particularly on mobile devices where browsers have less access to a powerful GPU/CPU, it's going to be costly to perform those resize operations there as well. So in short: yes, rescaling large images can be slow in Chrome sometimes. Try to prescale where possible and these performance bottlenecks should disappear.

jQuery.BlackAndWhite.js erratic on mobile?

I have been redesigning my WP portfolio website of late and have been playing with a neat little aesthetic jQuery plugin, BlackAndWhite, Which turns all of the images in my gallery view black and white and makes them recolor on hover using the HTML5 canvas.
I noticed one bug with the plugin is that if it runs before the images have finished loading, it will sometimes only grey-scale half of an image. So I have been using another tiny plugin to only run the script after I'm certain that the images have finished loading. see waitforimages.js
The initialization script is the following:
jQuery(document).ready(function(){
jQuery('.bwWrapper').waitForImages(function(){
jQuery(this).BlackAndWhite({
hoverEffect : true // default true
});
});
});
where bwWrapper is the wrapper around each img.
Now this works really nicely on most computers and browsers that I have tested thus far. Everything works the way it should. However, when I opened the site on my android, I noticed that only 50% of the images recolored. When I refresh the page, again only 50% go black and white, though it is different ones each time. Making the site fully responsive to mobile is still on my to-do list, but I have noticed this fairly blatant, yet difficult to explain bug.
Any idea what might be causing this?

You don't need to preload the images because the plug in aleady do it. Try to download the last version of the script on github https://github.com/GianlucaGuarini/jQuery.BlackAndWhite and if you still have problems send me a private mail with a link so I can check what could be the problem.

IE7 CPU spikes, script problems & debugging?

A website I'm currently designing displays and works perfectly in all mac browsers, and Windows Firefox, Chrome and IE 8.
I'm having major problems in IE 7 though. Whilst the CSS is pretty much there (a few tweaks needed), the site is maxing out my CPU at 100% rendering the site almost unusable and giving me scripting errors and javascript functionality is not working.
I don't even know where to start trying to find out what's maxing out the CPU, or how to get more info on the scripting messages - it tells me the line the problem is on but it doesn't tell me what file.
I'd like to provide a link but the client has asked me not to.
Any help would be greatly appreciated.
Thank you

Most likely, this issue is caused by ie's poor png rendering capabilities. In the situation that I have experienced, we were using jquery ui 1.8.7 to render modal dialogs and seeing the memory usage spike by 6-8 mega every time a dialog box was opened. It turns out that the culprit was the opacity and alpha CSS settings in the default jquery styles. See this post for a possible partial fix.
Other things to note include:
There seem to be many fixes that attempt to address this issue.
Jquery 1.8.16 has a partial fix where the large memory jump will only happen when the first dialog box is opened.
We have also experimented by setting a single pixel png as the modal background and this rendering of a single pixel caused the memory used by IE to spike 80 megs and caused a temporary spike in CPU usage.
One other peculiar observation was that if we set the modal value of the jquery dialog to false and created our own modal background by appending a div whose background was set to a semi-transparent png, the memory leak seemed to be smaller.
The short of it was to stop using transparent pngs and the opacity and alpha settings foe rendering in IE.