It took about 31 hours before it finally finished. Certainly won't do that again on the test server, next time will be on the production server for sure. I haven't checked, but I don't think I have that many really long threads.
Don't really know what kind of disks are in the test server, but the server is quite old so it's probably no speed freaks.
A CLI version of the importer script would be quite nice though, would feel a bit safer than having to rely on the browser, and it wouldn't need to be done in steps either.
Now I'm off to have a look at all the new features and to start tweaking.
