SQL Server: Slow restore from SQL 2014 DB, but Fast from SQL 2016 DB via SSMS
The restore from 2014 is an upgrade, not just a restore.  You should actually see this in the messages window, assuming you are using T-SQL to do the restore.   You don't mention what Fast and Slow are, or the differences, such as 5 mins vs. 5 hours, so we can't add much to this.