QUIC involves a lot more than copying packet data under a specific header so the bounds provided by memcpy() end up saying very little. E.g. QUIC mandates TLS 1.3, you'll have congestion and loss control (not every packet you send is guaranteed to make it), the tests are measuring goodput not network bandwidth, latencies for switching between/waiting for tasks to complete on a single core may be limiting.

Whether it's "considered fast" is better answered by simply comparing the practical numbers, like in that dashboard, rather than supposition. This kind of question is more aimed at trying to find out "how fast could it possibly be".