I always found it easier to improve the time waiting for postgres by tuning critical queries than to track down runaway memory usage and tune garbage collection. I also felt like I spent a lot of my wall clock time during active development waiting for apps to start up and run tests or whatever. There are best practices which speak to most of this, but they were hard won.
But that doesn't come for free with Rust. You still have optimize the DB. And then does it really matter which language you use, since the DB is still going consume most of the time generating a web page anyway, right?
My point was that I found the database to be the easier bottleneck to optimize. The other bottlenecks I mentioned, which I found more difficult to optimize - memory usage, GC, and startup time - are things for which Rust has a good story.
You should really be memcaching the database aggressively. I got it to the point that PHP was taking more time than the database for most pages because most pages DON'T need to be updated that frequently.