That is an unexpected behavior of the Tableau Desktop 10.5 .hyper feature. The extract should definitiely be at least somewhat faster but not slower.
Please open a case with the Tableau Technical Support teaml:
Please include as much detail as possible for the team to investigate. Including a copy of the original .tde file and .hyper file created would be fantastic.
Did you manage to find a solution to your problem?
I have a similar problem but with 2018.1. My twbx file with .tdes used to be 6GB and now with .hyper it is 61GB.
If you are having issues with the creation of the .hyper extract. I would attempt at using the following instructioins to upgrade the extract to .hyper from .tde.
Hope this helps!