QUIC, UDP buffers and the sysctls nobody tells you about

QUIC implementations like to ask the kernel for large socket buffers. On stock Debian the ceiling is around 208 KB, so the request is silently clamped and you get a warning in the logs, if you are lucky.

net.core.rmem_max = 16777216
net.core.wmem_max = 16777216

On a small VPS the measurable difference was modest but real under packet loss. Your mileage may vary; measure before and after.