PM6 open on screen iMac 27 (5120 x 2880) is slow.
If you open PM6 on the second display (1600 x 900), everything will work quickly. Flipping through previews and flipping through enlarged photos.
Leave the preview window on the small (second) display. and the window with the enlarged photo will transfer to the screen iMac 27 and start to scroll through the pictures. At the same time, the program starts working slowly again. But…
Press the "Z"button. the picture on the big monitor increases, and PM6 begins to scroll through the pictures quickly.
Miracles!
I think the problem is the high resolution of the display.