Waaaay too slow (have you ever used Subversion?) But
it might not be
a bad idea to put a WebDAV/DeltaV front end on whatever we create to
make it possible for third-party tools to access it.
I've used it since early betas, however not in a production
enviroment, just my personal repository, so I haven't felt much need
for speed, however as David pointed out you might get more speed out
of fsfs than dbd and I just mentioned svn as an example, there are
more version control systems in the world.
Regardless, using a VCS would bring diff-based storage, and when it's
all said and done making a custom implementation of vcs-like features
might end up in something not much faster or even slower than a "real"
version control system.