Incidentally that also solves the spam problem: for following people you don't have to worry as you'll only fetch stuff from people you follow. For spam on "global" timelines and in reply to other people's posts, you can easily limit your read to relays of a certain kind that provide some form of anti-spam control, and each relay can implement different techniques, some may use Lightning, others may use closed whitelists, others AI-filtering and so on.