The best I could find was a rough 2.5x speedup of ChaCha8 vs ChaCha20 in the "Too Much Crypto" paper and https://github.com/jedisct1/rust-aegis#benchmarks that benchmarks ChaCha20 and various AES implementations in addition to AEGIS, where it looks like ChaCha8 might be competitive with hardware-accelerated AES on some platforms (AMD) but not others (M1).
Unfortunately, this is comparing apples to oranges because AES-256-GCM is authenticated, so you will need a MAC with ChaCha12 (usually Poly1305) which finally makes ChaCha12 in AEAD mode slower than AES-256-GCM.
But the real question is: What is fast enough?
I believe that between 1 and 2 GB / s per core for an AEAD is fast enough as I/O will be your bottleneck way before that.
This is why I will always favor a ChaCha20/ChaCha12-based AEAD over AES and its many footguns.
On Pi5 AES GCM does 1.5-2GB/s per core, and ChaPoly is still something like 150-300MB/s.
You can do experiments yourself with `openssl speed`.
Security-wise they are roughly equivalent in practice if they are used properly.