I work on kernel and network performance on FreeBSD. For me the issue with QUIC is that we loose a few decades of optimizations in software and hardware. Eg, we loose TCP TSO and LRO (which reduce trips through the network stack, and thus per-byte overhead costs), and we loose inline crypto offload (which offloads the CPU and cuts memory bandwidth in half). So this makes QUIC roughly 3-4x as expensive in terms of CPU and memory bandwidth as opposed to TLS over TCP.