I struggle to imagine projects that can do with a db latency of several hundred ms? Something fully async maybe, any human interaction would be incredibly slow

And functions often have multiple db calls not just one. The latency would be a deal breaker no?

How far was the server from the Supabase instance?

I have a toy project with a server on Fly.io and the DB on Supabase within the same city

When sending API requests to the server, my end user latency was around 25ms-100ms (depending on the endpoint and how many DB calls it was doing)

Now with Supabase on Fly, that API latency is down to 17ms-70ms. But Supabase on Fly is still in alpha so it's not relevant for production yet

same aws region as supabase instance using a single query.

Same region, different provider. Also, their JS API has to do a round trip every time you set auth credentials on the backend, so it's at least 2 round trips for a single query.