I used to do multiple shrinks at once on a Q6600 to take advantage of multi-core.

For example, I'd be shrinking a 6-disc season of a television show and have it working on three or four at a time. I had to process each on a different HDD to keep multiple simultaneous R/W processes from slowing it all down.
Any idea is CloneDVD does this? I own it, but I've never used it (lost interest in DVDs when I started my BD collection).