It has nothing to do with the fact that the script is running. If you did a manual import, it would be just as slow. The fields are evaluating and also most likely building/updating indexes on them. You need the fields to evaluate and you probably need those indexes.
Isn't this a "one time" transfer of your data? If so, I'd just let it be and wait for completion. I've sometimes had to let an import records operation run overnight.
If you really want to prevent the fields from evaluating during import, make them unstored calculations before importing.
Thanks for the quick update. It should be a one-time script, however I know I'm going to need to run it at least one more time. This is for a live database and I needed actual data to rebuild a couple layouts and make sure they behave just as they did before the new table updates. Once I've made all the changes and have the new layouts working properly I will run the script again, but this time I'll run it after hours when there's no one else in the database making changes to record data.