That's too categorical a statement. It depends on a lot of things. Sure, there's a place in the world for PHP pages that take two or three full seconds to render, but it doesn't matter because they're doing something useful and don't get hit a thousand times per second. There's also a place for things that render in under a millisecond because they're getting hit at an incredible rate, and if they also have some sort of interdependencies then Rust can make a lot of sense.
Plus as I've said before, the whole "you're stuck in IO anyhow so who cares how fast the render code runs" dogma has sometimes gone too far. (I know you didn't say anything about that, but I'm guessing based on experience it's what lies behind your point.) My experience in switching from slow languages to fast languages, even doing the same IO, is that you do tend to see real performance gains unless your page was absolutely trivial. Slow languages really are slow in practice.