This is older than git 1.9, but git supports mercurial repositories[1]. I used to have to use mercurial to deal with kiln (which couldn't convert our repo to git), but now I happily work with git and everything is 10 times faster.
When was the last time you used Mercurial? Git being 10 times faster seems dated. Especially if you're using Facebook's fabulous Mercurial extensions, which actually make Mercurial faster than Git.
I'm curious: where does this 10x speed increase come from? are you implying that git is inherently faster than mercurial? I've not actually used mercurial myself so I wouldn't know...
[1]: https://felipec.wordpress.com/2012/11/13/git-remote-hg-bzr-2...