[api]

Mercurial works correctly but is unbelievably slow compared to git.

If you don't think about performance ahead of time (preferably at the algorithm level) then you will build something correct but slow and it'll be very hard to make it fast without redesigning major parts of it. It's possible to make design choices that paint you into a performance corner very easily, especially with something designed to work on a lot of data. O(n^2) might look like O(log n) for small data and in testing.