Well, one of the big problems in Monotone back in the day was that since they (we? I contributed a bit to Monotone, but was never a core contributor) broke peop

Well, one of the big problems in Monotone back in the day was that since they (we? I contributed a bit to Monotone, but was never a core contributor) broke people's repos several times with data integrity bugs, and so got very paranoid and added lots of slow integrity checks to make sure that didn't happen any more. Part of the reason for the breakage was that they were still experimenting with the basic representation of history, merge algorithms, and the like, and so had to do data migrations quite often, which sometimes introduced bugs. Git had the advantage that it could learn from Monotone's mistakes, and focus on speed, and not have to experiment as much since much of the basic design ideas were already there. Git also made several better architectural choices, I believe, going with a much more simple and stable repository format than Monotone had.