I'm working on a project with a Postgres database, and as it gets more complex I'm moving more stuff into stored procedures, pretty much wherever a single action requires multiple statements in series (e.g. check if this thing exists, check a value, get the id of some other thing, on success update another table).

Of course I would prefer to leave as much of it in the middle tier as possible because the ergonomics are better there but I don't want to sacrifice performance.

Yeah, if all of your queries are slow, you've got more work to do, but that doesn't change the overall process of optimization.