many of the clients implement the list of known users by users you have seen recently. far from perfect but it's a performance optimization. I think nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s has talked about this how host nostrdb project can help with this.
Discussion
It should at least look through our follow list. This list could be stored in the app cache.