The tasks that take a long time to restore are indeed multi-threaded. One caveat is that 2x the CPUs almost never is 2x the performance.
Other point is that usually the tasks for rake posts:rebake and the heavy lifting of the forum itself to recover and optimize the content can happen with the forum live. Which might reduce the time you need to have the forum either off-line or read-only and be able to offer a somewhat degraded experience.
My recommendation would be: first, test. Do the migration, see how much times it takes and how the forum looks without all the rebakes in place. If it’s good enough time the migration to end around the lower traffic time of your forum that way you earn some 4-10h of migration without a lot of people complaining.