Updating processor drivers fixed it for me, too
2.33 GHz C2D MacBook Pro 17 (Late 2006), 10.5.6, 3GB RAM, PD 4.0.3810 (assigned 1.4GB RAM), WinXP Pro w/all updates & SPs (assigned 1GB RAM, 256MB VRAM).
After upgrading to PD 4.0, performance suffered noticeably, accompanied by processor usage maxing out regardless of memory settings or use of one or two cores.
Upon seeing the suggestion to check for errant processor drivers in WinXP, I was indeed showing the yellow ! symbol and after some fiddling around, figured out how to update the drivers, which was successful.
I am now seeing quite reasonable processor usage, and the fans rarely come on anymore (before they would run almost constantly).
Speed in PD 4.0.3810 has picked up somewhat after this fix, but it still slower than PD 3.x. I'm exploring converting my expanding VM file to a standard one (was noticeably faster when I tested with an external drive), but haven't figured out how to make the standard file smaller (the conversion causes it to go from 38GB to over 100GB, which won't fit on my internal drive).
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