The biggest problem with mypy is that it's written in python. It's slow. It needs to be rewritten in rust or go to make it faster like how js ecosystem is moving in that direction. All the tooling is moved to faster compiled languages.
Those tools are not re-implementations of mypy, but their own type checkers. Pyre doesn't necessarily have all the features of mypy and vice versa.
Initial type checking with mypy is slow, but if you use mypy-daemon each check after that becomes pretty much instantaneous, even on 10M+ line code bases.