Ah, had EXACTLY this problem with P6 - and with some versions of P3DO too.

If you have a Logitech mouse driver, get the latest version, and then on the Motion pane UNcheck the new option "Disable acceleration in games." Apparently this feature - to add acceleration to games - was added several driver versions ago and caused slow downs in 3D apps using OpenGL. It wasn't until the latest version of the driver that they provide an option to revert to the older (pre '03) functionality.

Once I'd upgraded and unchecked, everything was fine in all applications - except my mouse was a little faster everywhere - so I had to reduce it from flat out where it had been forever to about 66%!