I tested both WebP and AVIF with maximum CPU usage/effort. I have not tried the faster settings because I wanted the highest quality for small size, but for similar quality WebP blew AVIF out of the water.

I also have both compiled with -O3 and -march=znver2 in GCC (same for rav1e's RUSTFLAGS) through my Gentoo profile.

Maximum CPU between those two libs is not really comparable though. But quality is subjective and it sounds like webp worked best for you! Just saying though, there is little benefit in using the max CPU settings for avif. That's like comparing max CPU settings on zip vs xz!