EDIT: actually, after a few minutes on their docs page, I'm still not sure "how it works" regarding perf, so this could be useful even for the not-so-lazy :-)
Writing docs is hard ...
You can think of PostgREST as one big pure(ish) function
It translates ONE http request to ONE SQL query and executes it in a transaction (after setting some context).
There is very little overhead. 90%+ of the time of a request is consumed by the db running the query. So usually, a request is about 5ms-10ms (without network time).