Pro is a lot faster than Desktop in most of what it does, but field calculating is still extremely painful on a large number of records. I know it isn't my PC. It exceeds the recommended system requirements.
In general, no, it is not "normal" for Pro to take 20+ minutes to calculate a field with that many records. That said, any discussion on the performance of calculating fields starts with sharing what the data source is because calculating fields with a local file geodatabase vs enterprise geodatabase vs hosted feature layer all perform very different.
That doesn't surprise me. I am guessing if you dumped the data to a FGDB and tested the same calculation, it would be < 30 seconds. If you are working with a version that has a very long/deep state tree, it can lead to significant performance degradation for activities like updating data. I would speak to someone who manages your EGDB and have them look at how many versions are outstanding and see if those versions can be reconciled and posted.
