Correct, you need to do everything that touches the UI on the main thread. But I never ran into problems that it would lead to frame drops, scrolling frame drop

Correct, you need to do everything that touches the UI on the main thread. But I never ran into problems that it would lead to frame drops, scrolling frame drops are because too large calculations are done (especially on the main thread) while displaying a new cell. I don't say it's trivial always to do complex stuff in a non-blocking way but if I can do it probably somebody working at Apple can do it too.