I am also having the same problem with PM 4.6.5: Folders with 200-300 image pairs (jpg+nef on local folder) do open extremely slowly (up to 30 sec) on my desktop PC (3 GB RAM, Win XP Pro, PM Cache settings like suggested).
My Antivirus software is AntiVir Premium.
I also use PM on an eeePC, again with AntiVir. Time to open same folder as mentioned above, but from external harddisk, is same as on desktop PC, despite much lower performance of eeePC.
Is a fix already available?
Kind regards
Holger Schulz