Migrating that amount of posts to anything is going to take awhile since it's limited by how fast it can read and parse the flat files. There is one option you could do if you have the hard drive space. You could close your forums long enough to basically make a copy of them. Reopen them, letting everyone know that anything posted after that point will not be in the new software. Then you can run the import tool on the backup copy. So your users could continue to use the forums while you were importing.
