We have a build where the compile step takes about 6 minutes and a checkout from scratch takes about 12, but the common checkout when no changes have occurred finishes in less than a minute.
We started using Clone Workspace SCM to try and reduce the redundant compilations, but what we find is that archiving the workspace takes about 7 minutes and pulling it down to a slave for a new build takes another 7. So, whereas we have successfully removed the redundant compilation, we haven't really improved the overall time it takes to get a build result. It just spends all the time copying now, rather than compiling.
Given that most commits are fairly small, some way to incrementally copy only the updates would cut the time down a lot. Someone here suggested using rsync to do the copies of the cloned workspace.