I have noticed this on both the iOS App and the Web App. I have folders with pictures and videos with several hundred items and when I open Emby to browse through them, it makes the app (or browser - either Safari or Firefox) freeze.
I was testing with a folder containing ~1700 items, and it takes a long time to render. And even when the page is completely rendered, scrolling is almost impossible.
As a short solution I was trying to enable paging back (setting the number of items per page), but I couldn't find the right setting in any of the js files on the server.
I read some threads from 2016 where people were complaining about paging, but with so many items, showing everything at once is also an issue.