The one important difference between Perforce and git (and the one most commonly mentioned) is their respective handling of huge binary files.
Like, for example, in this blog of an employee at a video game development company: http://corearchitecture.blogspot.com/2011/09/git-vs-perforce-from-game-development.html
However, the important thing is that, the speed difference between git and perforce, when you have a huge 6gb repository, containing everything from documentation to every binary ever built (and finally, oh yes! the actual source history), usually comes from the fact that huge companies tend to run Perforce, and so they set it up to offload all significant operations to the huge server bank in the basement.
This important advantage on Perforce's part comes only from a factor that has nothing whatsoever to do with Perforce, the fact that the company running it can afford said server bank.
And, anyway, in the end, Perforce and git are different products. Git was designed to be solely a VCS, and it does this far better than Perforce (in that it has more features, which are generally easier to use, in particular, in the words of another, branching in Perforce is like performing open-heart surgery, it should only be done by experts :P ) ( http://stevehanov.ca/blog/index.php?id=50 )
Any other benefits which companies that use Perforce gain have come merely because Perforce is not solely a VCS, it's also a fileserver, as well as having a host of other features for testing the performance of builds, etc.
Finally: Git being open-source and far more flexible to boot, it would not be so hard to patch git to offload important operations to a central server, running mounds of expensive hardware.
