Adobe has issued a Tech Note regarding poor Photoshop CS5 performance, slow and sluggish behavior, and generally unsatisfying performance and CS5 speed issues if you are using Optional Extension Plugins, especially in Mac OS.
In a bit of confusing info, the Tech Note seems to be referring to Mac OS, but also includes directions for resetting the preferences in Windows 7, Vista, and XP.
From the Photoshop CS5 Tech Note:
The way the Optional Extension Plug-ins for Photoshop CS5 are installed creates performance issues when running Photoshop CS5 on Mac OS.
Determine if you’re affected by this issue. If so, remove the plug-ins, reset your preferences, and then download and install the individual plug-ins you need, using the new plug-ins sets.
Please read the entire Tech Note, and any such procedure, before following the directions. As always, take care when updating.