As others have mentioned, there’d seem to be hard downsides:

• Can’t send direct links to people without an account
• Harder for people to ‘discover’ the instance or check it out, which would surely reduce people signing up to the instance and participating in our community

So I’d be against it unless there’s really strong reasons for it. I haven’t really experienced bad server-load issues.

There’s conceivably a middle-ground where the instance becomes logged-in-only automatically under high load. That’d no doubt be a considerable extra work, but just throwing it out there as an idea. I still don’t think even that’s necessarily a great idea, because again, if the traffic is people showing interest in the instance, that’s a good thing in the long term. It’d be better than setting it to logged-in-only permanently.

I think this should be pretty much a last resort for any performance issues, and any other measure should be considered first, including spending more money.