PM6 should allow you to start working with your images almost immediately and PM5 won't.
If you're waiting until you see "Sort Complete" in the bottom right corner of the window then I expect the two apps to be similar in speed. Sorting takes time either way. PM6 just doesn't block you from working while that time elapses.
I just installed the new update (build 3889) and have a few observations already. I'm not sure why, but now that the contact sheet is "locked" until sorting is finished, it actually sorts *much* faster than before. In previous builds it used to take sometimes 10-30 seconds to sort, while in PM5 it was a second max. Now it seems to be about the same time in both, at least on initial basic testing.
Also, even though you previously had access to the contact sheet while it was sorting, that never actually helped me in practice because after going through my photos once, I then view the tagged ones only, so when I would switch out of PM6 to do something, then go back in, it would always force a re-sort, and the view I got while sorting was all the photos unsorted, could not easily find any of the photos I actually wanted to work on during that 10-30 second refresh time so had to wait until it was done so it only displayed the photos I actually was interested in.
So great update, this actually makes PM6 usable for me again.