Can anybody point me to a reputable benchmark comparing this vs e.g Starlette etc?

My guess is that "fastest" refers to the request-response loop.

I'd be interested in knowing how fast it is once you tack your favourite template rendering engine on top.

It would be nice if there were drop-in support for Tera, the Rust template engine that is inspired by Jinja2:

https://github.com/Keats/tera