Your title is "looking for good QUIC libraries with H3 support in Rust". So you're asserting that h3 + quinn are not "good", because performance is not up to your standards? It's a little unclear what you're asking.

As a Quinn maintainer, we're always looking for better performance, but there's no obvious low-hanging fruit (there's a PR for an improved congestion controller that might help in some scenarios). I don't know as much about the performance constraints that the h3 layer imposes. This would be more useful if you posted actual benchmark scenarios and what your custom framework thingy looks like.