FB's terrible back functionality is their own fault since libraries like React Router give you a ton of control over back/forward behavior. As for FB's slownes

FB's terrible back functionality is their own fault since libraries like React Router give you a ton of control over back/forward behavior. As for FB's slowness, SPAs are hard to do well when you have a lot of live data all over the page. You can get clever with optimistic backend calls and caching, but in the end you still have frequent state changes. Users want infinite scroll to be fast, so you need over-eager loading _in case_ they scroll. Add over-eager loading all over the page (in case they click on a user profile, in case they click on FB Marketplace, etc.) and you have a recipe for a CPU hog.