Encryption is one thing (if you run kTLS which is still not done in most manual setups) but the much bigger IIRC is how much of the networking stack needs to run in userspace and has not been given the optimization love of TCP. If you compared non-kTLS h2 with non-kTLS h3 over a low-latency link the h2 connection could handle a lot more traffic compared to h3.

That is not to say that h3 does not have its place, but the networking stacks are not optimized for it yet.