I smell an implied claim of unfeasibility ... I'm fine with not getting events from people that are not followed by any of my follows. Any smartphone can store that list. If my follows reply to a note, yeah, load that, too etc. I don't see CPU or RAM to be a limiting factor.
If you only consume the content from your follows you are indeed safe but thats not a social network experience If you want to potentially be exposed to content of follows of your follows its approximately 100k-100kk users which kinda could work on a moderate gaming pc but def not a regular tier-2/tier-3 smartphone If you want to see more 🤝🤝🤝friends of friends connections you need a solid hardware cluster given that at least some activity happens
100k-1kk* of course
Not enough for temporal responsibility. Add a zero tolerance.
The follows of follows is a very moderate number and even if it were a million, that's very much manageable. You throw them into a cockoo filter that's far less than 1MB and you can filter by that set trivially in RAM and CPU.