Since some days I suddenly get freezes in the tab; these unfreeze after some time, say 20 seconds or so, but I notice a delay.

Something has really changed to the worse lately. I think it has to do with anti-ad programs as well as AI, like the UI also changed. It is important to point out that while you do not have any issues, other people do or may.

I've been seeing weird stuff where the video will play but the page is unresponsive: can't click to pause, can't click the time index bar to move to a different point in the video, etc. Then I close the tab, and the audio keeps playing! It will play for 10-15 seconds before cutting out. Bizarre. . .

I had that happen some week ago too! I had to clear site data + cookies for YouTube in Firefox before I got it to work again.