It's really only an issue on mobile when you're collapsing larger threads. Try collapsing the first comment in this thread, for example: https://news.ycombinator.com/item?id=14656945

Not terrible, but there's a noticeable (maybe ~1 second on my phone) delay. My desktop is fast enough that it still feels instant.

Same. I am really picky about performance, and I've never seen anything but instantaneous response from collapsing comments. I just tried it right now on a few example threads from the home page and confirmed that it's instantaneous. I would estimate less than 50ms. I can rapidly click a + and - and collapse/expand a comment tree more or less as fast as I can click (the limiting factor being that my browser wants to select the + button itself as text).

For whatever it's worth, I use Firefox on Windows. But I just fired up Chrome and gave it a test, and I see the same thing there. But I don't do a whole lot of HN comment browsing on my mobile device, so I suppose that's the difference.