This is a straightforward verifier for Metamath files written in Lean 4, a new dependently typed functional programming language. As of this writing it will compile set.mm in around 10 seconds.
To compile the verifier, you need to acquire lean, and it is recommended to use elan for this since Lean 4 is still rapidly changing and this version builds on a nightly release of Lean 4. Once you have it, you should be able to run
elan override set leanprover/lean4:nightly
elan update leanprover/lean4:nightly
leanmake bin
and it will produce the binary as build/bin/Metamath, which can be run as:
build/bin/Metamath path/to/set.mm
to compile set.mm in the current folder.