I have been using GitHub since 2011 and it's undeniable that the performance of the website have been getting worse. The new features that are constantly being added are certainly a factor, but I think the switch to client-side rendering that obviously shifted the load from their server to our browsers and also tend to produce ridiculously large and inefficient DOMs[1] is the main cause.

If you want a practical example, here you go. I'm a Nixpkgs commiter and every time I make a pull request that backports some change to the stable branch, GitHub unprompted starts comparing my PR against master. If I'm not fast enough to switch the target branch within a couple of seconds it literally freezes the browser tab and I may have to force quit it. Yes, the diff is large, but this is not acceptable, and more importantly, it didn't happen a few years ago.

[1]: https://github.com/orgs/community/discussions/111001