Poor design and architecture. Doing too much and blocking the event loop, memory leaks, using too much frontend code to poorly mimic native browser features like new tabs and back buttons, uncaught exceptions that stop the javascript thread and leave the page stuck, etc.
React ecosystem is centered heavily on "components". This sounds nice, but IMO it breaks down quickly on the "whole program" level. What you end up with is crazy re-renders because it doesn't make sense to "componentize" the state.
Facebook absolutely is, it's a beast. I find it can frequently freeze up my old laptop, which is admittedly getting past it but I have little trouble with most any other site.
