yeah, that would be cool to have a specialised relay that just spiders the network constantly and only stores some small set of event kinds like this (wolud make sense to put profiles, follows and mutes, at least in one bundle) - then you literally can just have high confidence just from one query on it and done

Yes. Can be done. For user search I actually pull all profiles from npubs from huge relays like damus and primal in a local database. Could also get their follow lists and then from there you could query pretty fast for followers etc. will take some time for initial sync and you need to update every couple of minutes but then it should work more or less fast locally.

thanks for the provocation though, i'm going to do something i meant to do for ages, which is to apply maximum possible compression to follow/mute list storage, by creating an npub index with a monotonic counter value, then i could add a profile/follow/mute list spider that just gathers as many of these as it can find during the spider runs... commonly follow lists in particular are in the hundreds of kilobytes in size, with this optimization i can squash that down like tens of kilobytes per user, and thus store ten times as much of such events