Of course location is not a guarantee of fast speeds, but you can at least check servers nearby with https://lemmy.fediverse.observer/map. I guess you can check for uptime to have an idea about reliability, but I never really relied on that. Shit happens (vlemmy comes to mind) so it's hard to gauge an instance's future reliability/stability. You have the option to start your own instance as well.

https://github.com/CMahaff/lasim