I have been using Lemmy for 20 days, at first I opened an account at Lemmy.world because you can join without writing a text and waiting approval. I have been enjoying the experience overall but despite the admin teans best efforts Lemmy.world has been experiencing some serious performance issues. If you want to avoid that join a smaller instance, preferably hosted in your country. I joined discuss.tchncs.de today and everything is so much faster it has added benefit of being able to see beehaw.org posts too. It will improve not only your but all other Lemmy.world users experience too.
Servers are federated by default so unless it has been defederated by big instances you will have no problems. Important thing is instance you are joining having captcha/email verification because the ones without is suspectible to bot sign ups. So big instance might defederate with them to prevent bot attacks