not OP, and indeed git is fast-enough in many cases, but git not cutting it at Google and Facebook scale, combined with the versatility of mercurial (monkeypatching and extensions system) was the reason why they both invested heavily in mercurial instead of git.

Among the tricks being used was remotefilelogs, which is a way to "hydrate" content locally on-demand, which was mimicked in git many years later with Microsoft's git-vfs. Same goes with binary/large files that git eventually got as git-lfs.

It's funny to think that a big reason for git to be "fast" today is by playing catch-up with mercurial, which carries this "forever stigma" of being slow.