I'm just newbie in Rust. Does some web framework not use hyper? I'm trying to find web framework that does not have problem with Slow-Loris.

This doesn't solve your problem out of the box, but I think most reverse-proxies have configurations that can protect the services behind them from these sorts of attacks.

I don't think anyone will want to avoid hyper, tbh, that's a lot of work to hand-roll instead. so you better advertise the issue there instead, and if they refuse to do anything about it then make a big stink downstream.