I quoted this elsewhere but this part makes it sound like they are intending to move over large parts of the implementation into rust in the long run, migrating away from python due to the size of the project becoming more difficult to maintain

> In addition to performance concerns, Python is also hindering us because it is a dynamic programming language. Mercurial is a large project by Python standards. Large projects are harder to maintain. Using a statically typed programming language that finds bugs at compile time will enable us to make wide-sweeping changes more fearlessly. This will improve Mercurial's development velocity.