No, trouble started while ingesting.
In my previous post I stated: Another observation: Older folders, that were previously ingested, annotated and browsed through by PM5 run smoothly on PM6 too. Unfortunately, that is not always true, this morning I started browsing through a few folders from last year, and it took 2 seconds per image...
Curiously, after a while (didn't time, but at least 15 minutes) and after messing around a bit in the preferences and closing and opening folders, PM6 suddenly ran smoothly, and from that moment I couldn't re-create the slowness from the beginning; even after rebooting the computer, and having have 8 folders open, everything ran fine.
One thing I remember doing was setting the High DPI Display Quality to Standard, both in Contact Sheet and Preview preference panes. I believe starting from that point things got better. Later I switched the settings to Best again, but PM kept running smoothly, even after the reboot.
Hope my observations may help. Will now switch off my computer for a few hours; see what happens when I fire it up again...