Rather for myself, but as an account setting so that I don't have to collapse them manually for each thread. Actually it stems from something that bothers me with HN: when there are a lot of nested replies, I lose track of the which comment corresponds to which parent. For example, try reading this thread : https://news.ycombinator.com/item?id=22705222, I find it difficult.
If there are 5 comments (level 0), that each has 10 replies (level 1) and each one of them has 10 replies (level 2), I wish I was able to collapse level n>0 replies to read the comments first, then be able to unfold level 1 replies for the first comment, while keeping level 2 hidden, and so on. This might not be the ideal solution, maybe simply the way you added arrows to replies would make the above thread easier to read.
> What things lag the most? (Some specific button(s) or animation or scrolling or ... ?)
I just found it's not specific to android (I'm using Android 10 on a recent galaxy smartphone). Mostly animations. On the desktop version, If I click on the plus button near 'Chat Channels', the login window is show progressively, and the background is dimmed in way that 'feels' slow. Also when I unfold a collapsed reply, the page moves slowly (is there a reason it moves ?). When I click reply, a window is shown, 'loading draft...' appears then disappears quickly and then the preview is highlighted, it somehow feels slow.