Going into full-screen mode sometimes takes 30 seconds; it'll keep playing but show a black non-full screen and an empty UI. It randomly skips to other videos when I try to pause the current video. I could make countless other little nitpicks. The search bar's text isn't vertically aligned when searching for people's videos on their channel, etc.
It feels like they introduced a bunch of blocking code in the event loop and the UI is just slower than a turtle now. This happened recently, too. YouTube has otherwise been fine on this computer for a really long time.
They switched to JS page transitions instead of a full page reload, but it's actually slower. I end up always hitting refresh right after clicking on a video and the URL bar updates and a full refresh is still faster than their JS-based one.
The entire page just feels heavy, and I'm afraid of clicking anywhere as to not disturb the page and cause a reload.
Fortunately there's a third-party web frontend for YouTube that doesn't have these problems: https://invidio.us