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.
snap 888 for me, so 1.3x cpu and 2.6x gpu according to some random benchmark site. Older upper-range phone, also 12GB ram.
Must be my resolution/screen density then. My display width is 1644, so 2.3x the pixels of a 1080p screen. The android "density" value is decreased to 525, the 1080p equivalent is 345. That makes stuff a lot smaller on my display. I think the issue was with the images being scaled down a lot, more so than their actual size, since like ten 500² images should be no issue at all.
It's probably the dpi decrease rendering the images even smaller making it worse in my case, tho the resolution prolly aint helpin.