tl;dr — Enable the hg inotify extension to make hg faster.

People often complain on #developers that Mercurial is slow compared to Git. But if you run Linux, you may be able to make hg much faster by enabling the inotify extension. (You know the drill: Add inotify= under [extensions] in your ~/.hgrc.)

The Mercurial wiki ominously warns that this extension is "definitely considered experimental", whatever that means. I've been using inotify for a few weeks now without any problems. (I regularly back up my patch queue to a Mozilla user repository, so as to limit the possible damage if something goes kaboom.)

And man, is it fast.