A pseudo-meme I've noticed recently has been the discussion of Git and Subversion and their issues when it comes to large projects. It would seem that a repository of 6 GBs or more is too much for the scions of open source SCM. Steve Hanov has a detailed discussion of the problem on his blog. In almost all of the discussions I have seen in the past few weeks, Perforce comes up as the SCM offered as the solution.
There is a head-to-head comparison over on Stack Overflow where some folks compare Perforce and Subversion. It seems the real issue here is simply the way Subversion and Git handle their repositories on disk. When you're dealing with huge trees in Subversion, with thousands of files to move around or merge, it takes a non-trivial amount of time.
Hanov says that Perforce was confronted with this type of problem years ago, by Google. There's a link to a rather large PDF linked to on his blog, but I shan't link to it here due to its size. It's a good read, and gets into some nice juicy details about speeding up access to a large repository.