Mercurial is not slow. I switched from Mercurial (also called hg if you find that more catchy) to using git, but only because there are a few corner cases where it is more flexible. The downside is git got obscure commands you need to type in with ambiguities, 2 commands for same thing and 1 command doing 3 things.
hg you can learn in a day with easy commands you can even shorten when you type them in and rarely needs extra switches. Command names also have similar meaning as in SVN if you switch from it. Turtoise hg is also nice, I think they programmed it from scratch for hg only using the pictures of Turtoise SVN.
git you need to study and google for commands and dozens of switches per command for years to come. If you used SVN first, google every command, because all got intentionally renamed and do something else if you type in the original command. Turtoise git you better not use, I think that got into live as an awkward mutation of Turtoise SVN and did not fit git well.
The good thing of both compared to SVN is you can easily setup a repository, have everything locally on your computer without needing constant access to a server, you can easily setup a repository on an USB stick and push from your HDD to it to have a local backup in case the HDD or the server blows up and merging (and you can additionally rebase) is so much easier.
Edited by wintertime, 15 November 2013 - 09:23 AM.