> Performance optimization has become a lost art.

It's really not very hard, basically add indexes wherever lookups are done but don't go overboard, especially with tables that see a lot of inserts. The DB will tell you what it's doing if you ask, it's all easily investigated. If your joins resist optimisation your data structure is probably off.

> I won't even touch on proper design, which requires someone with significant database expertise from the get-go

In more extreme cases this is true, but a few basic principles and an understanding of databases will get you 95% of the way there and is quite intuitive. Something as basic as "don't copy data around, each individual entity exists in one place, and use its key to refer to it otherwise" will help enormously.