I understand that some application are not optimized, a good GUI widget would render only the visible part and a buffer , the fact that we don't have such good library or even beter native component in HTML is sad , especially that people are making desktop apps with electron. What is more sad is trying to excuse this with some fake ideas that pagination and bad implemented infinity scrolling is superior to optimized GUIs.
Just in case I am not clear. Say you have an Array with 10k image thumbnail urls and you want to show them in a Grid, this Array fits in memory so paginating using network request is stupid and makes things slow. So if all images urls are in memory you can create 10k IMG elemetns because the DOM is slow(especially if you have some DIvs and spans to wrape elements around, center some img name , maybe a button or menu). So if on my screen I can fit 15 items , I could create 30 img elements and as you scroll you would move the UI items that are no longer visible at the bottom and change the attributes like src, name etc. This is not simple and decent toolkits do this for you by default similar how a text editor can load a large file fast because is not painting all of the file text.