Substack is a good example as well. Literally cannot render simple text and image articles without visible and annoying lags. If there are several hundred comments it can take tens of seconds to finish rendering. With functionality that was figured out in early 90s...
I remember using Youtube with Opera GX and two adblockers not that long ago. When you scrolled deep enough in to comments writing replies to them had significant lag on input. And I mean like what felt like seconds before all presses got processed.
This is flagship product of one of the largest companies, and even they cannot get UI performance right...
