Hg is marginally slower, but not so you'd really notice. If you're crunching 20,000-line files on a regular basis it might be a concern. On repos of that size, I doubt you'd notice.
Git is a great tool for what it was built for and the group of users it was intended for--that is, rapid, tons-of-merges-and-pulls development on a gigantic codebase, for users for whom it is acceptable for man pages to serve as reminders rather than instructions. However, I appreciate the additional attention paid to tooling and user-friendliness in Mercurial-land that make it more pleasant for me to use.
The fact that all the tools can push to github make it easy to switch, which I just might do again. bzr was great, except for the fact that it just doesn't have that much traction...
I found bzr to be much as you described it - like hg, but slower. Nothing objectionable about the workflow (though the tools on Windows were very poor, I wasn't using Windows much at the time), just the perf on a particularly large chunk of code.
Git is a great tool for what it was built for and the group of users it was intended for--that is, rapid, tons-of-merges-and-pulls development on a gigantic codebase, for users for whom it is acceptable for man pages to serve as reminders rather than instructions. However, I appreciate the additional attention paid to tooling and user-friendliness in Mercurial-land that make it more pleasant for me to use.