The only reason QUIC delivers throughput improvements over TCP is because it uses the same congestion control as BBR, so occasional packet loss doesn't kneecap the connection. If you use BBR TCP (or RACK TCP) on FreeBSD, you'll see the same improvement in throughput vs older TCP congestion control.

The same server that will do 375Gb/s at close to 50% idle will maybe deliver 70-90Gb/s of QUIC with the CPU maxed.

TLS+TCP delivers that performance with a lot of optimizations that just don't exist for QUIC:

- inline TLS offload (Mellanox CX6DX)

- async sendfile -- note, the above 2 mean that the kernel never even maps data from a file being sent to a client into memory, much less copies it to/from userspace like with QUIC. This cuts memory bandwidth to roughly 1/4 of what it would be with a traditional read/encrypt/write to socket server.

- TCP segmentation offload

- TCP & IP checksum offload

- TCP large receive offload

Some of these optimizations are slowly becoming available, but until all are present, QUIC will cost 2x - 4x as much to serve as TCP for a CDN workload.