This issue is also important to me, as at my company we have a very large git repo with over 30 submodules. Cloning speeds, even with reference repositories, are quite slow (especially on Windows). We're talking ~10 minutes just for the checkout step, but I have done some basic tests and we estimate we can get this down to just a minute or two with a combination of reference repositories and updating submodules in parallel.
Mark Waite I am sympathetic towards your workload and understand that you have other priorities. I've done some Java development in the past, and I poked around the git-plugin and git-client-plugin source code. I think I've come up with a solution, but I'll probably need some help getting it merged since it's my first contribution to any Jenkins plugin. Your help and patience would be appreciated!
Let's continue this discussion at GitHub, where I will submit a pull request soon.