After some help from Support, I can use the 4.5.6 GroupWise Migration Wizard to migrate accounts into Zimbra. I migrated two small ones, which went ok, but now I'm doing a larger one - approximately 23,000 items. But the import process is glacially slow - it's been 4.5 hours and it still hasn't finished with the 2nd (large) account. Frankly, it was slow with the others, but they weren't that big, so it wasn't as obvious (still took quite a bit of time, but more like 30-40 minutes).
What sort of speed should I expect to see? What is it most dependent on? Can I disable indexing during the account migration, and would that help? What else should I expect to see?
Disclosure: The server load is not high, but I will admit that this is a VMWare instance on a relatively low-powered server. Even so, load avg. is about 0.5