Kirk: Maybe speed issue(s) are not just with Mojave.
I really like the new Import from Selection option and also revealing contents of folders in the Favorites pane is a lot faster in PM6.
Unfortunately, PM6 is otherwise too slow for me. And I'm running High Sierra 10.13.6.
Example: I open today's 1,681 images in a browser and then open a preview of the first image. I then press the right arrow to scroll through previews of all 1,681 images. In PM6, it takes 5 minutes 43 seconds to get to the last image. In PM5 it only takes 2 minutes 30 seconds. So PM 6 is more than twice as slow. Plus PM6 can't keep up with the image preview generation. Towards the end the previews are just empty whereas PM5 keeps up just fine.
15" MacBook Pro
MacBookPro15,1
Intel Core i9
2.9 GHz
Memory: 32 GB
2TB Apple SSD
High Sierra 10.13.6
Please let me know if I can help. I did request to be included in the PM6 beta but was denied by Patrick Wilson.