There were three distributed VCS that were in the running for a while. There was git, bazaar (bzr), and mercurial (hg). For a while people were using on all 3, but eventually git won out.
In my little experience with bzr, it was slow as molasses. The only competitive advantage it had against Git at the time was a simpler command line and better windows support (we are talking about the days when git commands were still spelled like git-branch, git-reflog and so on.)
Eventually Mercurial ate Bazaar's pie and now Git is slowly eroding projects away from Mercurial. All hail Linus, who managed to create two open source systems that became de-facto industry standards.
How does it compare speed-wise with git? That was the reason I switched from darcs 8 years ago. Some of my repos go back over a decade, and darcs had problems with them when there was just two years of history.
There was some issue with exponential running times in certain scenarios. I ran into an issue where two repositories were somehow in a state where a pull would never terminate. darcs definitely had problems.