Take this comment as an example: https://lemmy.dbzer0.com/comment/20455448
There are a few images with notable resolution (500×500) in that vote comment, that are shown at very small size. Whenever they come into view, the fps of the entire app drops notably for me, down to perhaps the tens. I've seen it go even lower on other votes with more emoji.
My phone is quite powerful, and browsers for example can display similar cases fluently even on much worse hardware.
My phone's resolution is 2412×1080. Devcheck says 480dpi. I think there could be some other issue.
That checks out, 1080p is normal, and 480dpi would be 730 at my resolution, so my 525 makes the images 40% smaller rendering over a similar number of pixels (10% more).
I guess this was about the scale then, how little of the total display the image was taking up.