Chat box on youtube live streams takes a while to load compared to other browsers, it takes an abnormal amount of time, this was with ishowspeeds chat, it can be replicated by just watching his latest live, the chat replay also does the same, not sure if this applies to smaller streamers with quieter chats.
When watching twitch streams or videos on youtube some times the whole UI around the video bugs out and shows my browser wallpaper in the blurred state (the blur that applies to the wallpaper at the top by the search bar and tabs etc...) It flashes, I'm not really sure how to replicate the issue, it just happens. The video is fine it's just the websites UI around it.
Overall I love the UI, not really here for the mindfulness features, I saw the design and decided to try it. Everything is fairly responsive. Sidebar opening animation when it's set to "auto hide" could be smoother as well as when opening the easy setup menu at the top right, that feels a bit "unsmooth" now and then.
Another thing with the sidebar, when it's on "auto hide" I think the areas on the screen below and above it should be clickable to make it more responsive. And a dark mode would be nice, as I prefer it and have to set sites to dark mode manually if they support it.