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White flickering/flashing when changing tab in chrome to react site

Hello StackOverflow 😀​
I'm having a strange problem with a react app when switching tabs in chrome (the app is already loaded). example : (Link to video if the gif is too low quality)
You can see there is a small white flash before it shows the site itself (something like 0.2 sec), the thing is my app is a little heavier I guess and it's sometimes 0.5-0.75 sec of white screen flash like that which is annoying customers.
Some of them describe it: 'the web page being blank for a 0.5 sec every time we go to another tab in the web browser and we get back to your app.'
I have seen some sites that have the same issue, for example, instacart.com, some of them have 0.1-sec white flash, and some of them have longer flash.
My question is how can I improve this? and what is related to this?
Most of the questions here are related to some stuff that is in react-native, but my app is react web.
I have read about FOUC but I'm not really sure if it's the issue.
btw I don't think it's related to the power of the computer (I'm getting this on a ryzen9 PC and M1 pro mac with 32 GB ram).
Thanks for help.
Also, this problem seems to only exist in chrome, in firefox it doesn't have any white flash. I guess it's related to this (see the first answer). How can I improve it?

The reason may vary. There are several ways to assess and solve performance issue(or flickering issue) with your web app.
Here's what comes to my mind:
Environment check before troubleshooting an web app
I'm getting this on a ryzen9 PC and M1 pro mac with 32 GB ram
The machine spec only holds some portion of performance assessment. It may or may not matter. because,
OS can slow down the performance due to its update issue. what OS are you using?
Web browser can cause the slowness as well. what internet browser are you using? and what version of browser is it? are you using any specific plugin or extension that may cause trouble?
So when assessing or QAing an web app, it is necessary to describe every little detail. When I was developing a Vue web app back in 2019 in a startup, there actually have been some real performance issues in production deployed environment that are bound to specific version of browser; not just that, one time I had a chrome extension causing a crash for my web app as well. Always make the reproducible environment clear unless it is exact which code section is causing the problem.
Once made the details clear, try to reproduce the problem. Does it reproduce in different browser, different machine, different OS? if not, environment is clearly not an issue.
Finding Problem with (any) Web App
If these little details are not causing the problem, it's time to get metrics inside browser.
I had no choice but to skip the environment assessment because the setting isn't clear in the question. But please do not skip that part; it is crucial. The problem might be fixable in current environment but it can remain in different environment.
Somebody already mentioned React devtool profiling in comment but I also recommend these tools:
Chrome Performance Tab
Chrome Lighthouse(previously Chrome Audit Tab)
Please try run a performance check with these tools first.
I've run a performance recording from Chrome Performance Tab and limited the scope to the flashing moment - and a slight moment afterwards.
(screenshots inside red lines are indicating white screen flashing moment)
Most Probable Reasons at the Moment
according to Chrome Performance Tab + Lighthouse audit, these problems are existing in that flashing moment.
treeshaking/code splitting is not used properly: whopping 40000 lines of code in a one file!(content.js) it should served in smaller chunks, so that the codes not important at first rendering must be loaded only when needed. check your code splitting setting first.
lighthouse also highlighted unused codes in a big chunk of single file are the biggest reason for performance dragging in your site before FCP(First Contentful Paint).
too many third party scripts: every third party code is casually lying around in the page with <script /> tag, without any performance tweaks. there are far too many third party libs running. they are not blocking the main thread as they are in async mode, but loading too many of them in parallel is definitely slowing the app. try load third party libs only when needed by inserting them dynamically.
this is most probable reason to my eyes but we need to dive deeper before drawing any conclusions.

Is it possible that this is related to Hardware Acceleration? I usually turn this off in my browsers, and I do not experience what you're explaining on the site you provided (I'm running Chrome on a Macbook Pro 2020 13" intel).
Perhaps turning Hardware Acceleration off will fix the issue?
You said that the issue only appears in Chrome, do you have any extensions that could affect the page when switching tabs? I'm thinking any extensions that reads or edits the page in any form or way.
I guess you have already tried this, but if not: could re-installing chrome and testing the sites with a fresh install (no extensions, no profile logged in...) work?

So in the end the solution was pretty crazy, we had a picture with the logo of our company in the navbar (which is appearing on each page of the app).
The problem with the picture was - whopping 35,000 px width and 5,000 px height.
You couldn't see it because it was inside a div with a fixed height and width.
The way I found that - opened dev tools, and started to delete the divs.
for example, we have:
<div id="root">
<div>1</div>
<div>2</div>
</div>
So I delete div 1, then check if the problem is still there. if the problem is still there I go delete div 2. And continue like that till you find something crazy like a 32,000px picture.
Very 'old school' but it is what it is (:

Trying to speed up load time on my website

I am in the process of trying to speed up my HTML website and the first thing I have done is to reduce the images to the exact size that they are loaded into.
My PageSpeed Insight for desktop is 90 which I am very happy with but for mobile, it is 24, which isn't so good (was 19 before I did the images)! It says that I can save 4 seconds if I "remove unused JavaScript" but I haven't added any JavaScript to my website yet (only just taught myself HTML and CSS so moving onto JavaScript soon).
So I am wondering what this unused JavaScript is and how I can remove it if I wasn't the one to add it in there in the first place. Any other tips on how to speed up the website will be much appreciated!
Thank you!

If by "speed up the page" you mean improve the performances of your website:
Changing the dimension of images don't necessary mean that they are "optimized for the web". You can find tools like https://optimage.app/ and information on the web on how to compress them.
You can use the inspector of your browser (Right-click on the page + Inspect) to do this :
Go to Network Tab, clear all logged requests then reload the page you will be able to see which requests are the most time-expensives and maybe do something about it.
Go to performance, start recording, reproduce the actions that you want to analyze
(You can zoom in and out in the timeline and click on the element to have more details)

Delete all Javascript from your website

Is there a way to get the script who's blocking browser?

We have a large old web application organized with iframes (many), every iframe is a html page (to be more specific asp.net webform, but the problem is on client side).
We have a problem with some javascript but we don't know which script and in which page.
When the user navigate to a specific section of the web application, the application freeze. Browsers generally alert the user that there is a script with long execution, and suggest to quit the script.
We can't find where is the problem. Is there some feature in modern browser to step directly in developer tools of the browser, in debug mode on the script who has the problem (instead of simply quit the script) when we face the problem, or some other workaround to detect the script?

The comments on my answer helped for a very simple solution I didn't thinked before... Keeping the debugger running I noted on debugger toolbar (IE, but suppose every browsers work the same) the buttons "pause" (2 vertical red sticks) and "continue" (green triangle). Pressing pause the debugger stop on the exact line the script is running in that moment...

Why would Chrome take 12 seconds to do an Update Layer Tree?

I have a relatively un-complex ASP.Net application, which occasionally displays an alert message in layer, with a shadow, with a pushbutton to dismiss the notification.
This code has been working fine for years.
Lately, I am getting complaints from users of really slow response. What they experience is that the page is completely unresponsive, and when they click on the [OK] pushbutton nothing happens for a really long time.
When I record a Timeline, I can see the original even which caused a round-trip to the webserver, and I can see the response from the webserver. I can also see all the Javascript code which runs as a result of the response.
As far as the application is concerned, the transaction is over, and, as far as the Javascript is concerned, the transaction is over as well. All we need to do is wait for the user to click the [OK] button and we will dismiss the notification popup.
This is where the "freeze" happens. The [OK] event is not delivered for like 20-30 seconds. What's going on in the meantime? That's why I'm writing:
We see some very short timers fire every once in a while, then we see a quick Recalculate Style call, followed by an Update Layer Tree event.
The Update Layer Tree event takes 10-12 seconds!
Here's the code for the dialog which is going unresponsive because of the Update Layer Tree events The gets displayed first as you see it here, then, if something goes wrong, it may get updated with code similar to showAlert:
Status/error dialog code

See #tiblu's comment for detail on why Chrome would perform Update Layer Tree.
As for taking 12 seconds: does the interaction behave normally in other browsers, for example Firefox or Safari? If your code has been working fine for years as you say and the browser response issues have been popping up recently (> mid-October, 2015) and only in Chrome the issue may not be your code.
There are a number of similar new issues and complaints recently, pointing to the release of Chrome 46. For example: this issue (code.google.com) and this issue (code.google.com).
CPU spikes are responsible for browser unresponsiveness, which you can monitor using Chrome's Task Manager. As for why CPU is spiking and further detail you'll need to follow the issues above and others as they develop.
Apologies for the pseudo-answer, don't currently have the rep to slip this into a comment.

Thanks to all who responded ... I was unable to figure out how to re-install an older version of Chrome, but I did manage to get my hands on a beta version of Chrome 48 (48.0.2564.8), and, to my most pleasant surprise, I am no longer able to reproduce the problem.
Just to be sure, I restored 46 from this morning's BACKUP, and it re-exhibited the issue. After re-installing beta-48, the problem is gone.
Hooray! Well done everyone!

Find conflicting Javascript with browser developer tools?

I'm trying to debug a CMS-backed website (Drupal 7). It is displaying some problems due to conflicting Javascript. For a fraction of a second, the site components are fine, and then *SNAP*, they're broken. It must brake as soon as the conflicting resource loads.
Using Chrome's developer tools (or Safari, Firefox... even IE's), is there a way to load the page, one Javascript resource at a time? This way I can see exactly which resource loads when the page issues appear.
(I'm also open to better ways of debugging this kind of thing.)

You can use the Pause on exceptions feature of DevTools, in Sources Tab, it is the hexagonal pause button at the far right.