That does seem like a slightly excessive time. Has this error persisted when connecting to .hyper files that have been created by Tableau Prep?
I would also be curious on the versions of Tableau Prep and Tableau Desktop that are in use? If you could please provide that information.
The slowness was occuring in version 2018.3. I've upgraded to 2019.1 and the problem seems to have been fixed. However, I'm now having issues with running out of memory, as well as the following: