I'm working this problem right now. I have an importer from 6.5.5 to 7.1 that did 4,000,000 posts in something like 19 hours. Unfortunately it had a minor bug which I'm fixing now (and then I'll have to test, etc.) My plan is to make it available to the community.
When I tried to import this same table using the normal importer I gave up after 3 days of it running on an empty server.
So was the database on a different server than the importer? If so, I think that could cause the slow import times since the importer is pretty much 30mill queries in your case
