High-performance vanity gen

I was attempting to run `chantools vanitygen` on an AWS m5.24xlarge using this command:
`chantools vanitygen --prefix 0212345678 --threads 255`
and I only got about a 5x speed improvement over a quad-core laptop.

Any ideas?

This instance should have 96 cores. So was hoping for like a 10-15x improvement.

Start by reproducing `chantools vanitygen --prefix 0212345678 --threads 255` on the reported AWS m5.24xlarge and compare its throughput with the quad-core laptop result. The issue is complete when the cause of the limited speedup is identified and the vanity generator's scaling is improved or clearly documented.

Written by the indexing model from the issue text.