At the bottom of the image window in Photoshop it is possible to set the fly-out menu to "Efficiency".
This is a displays the amount of processing that Photoshop is doing in RAM. 100% is ideal, if the figure starts to drop below 75%, you will notice a significant performance slump.
Turn this guy on and monitor the % figure, if it frequently drops from 100%, it might be time to allocate more Ram to Photoshop.
When Photoshop runs out of Ram to perform operations, it moves to writing data to/from the Hard Drive. This is known in Photoshop as a "Scratch Disk". Scratch Disks are configured through the Preferences in Photoshop. (Photoshop CS allows for up to 4 different disks (partitions) to be nominated.)
For optimal performance in PS, create a separate partition on your HD and call it "Scratch" (or similar) nominate this as the primary Scratch partition. Using the default startup disk for PS results in poor performance as the boot disk is usually fragmented. (This may be different under MacOS 10.x but is certainly the case under 9.x)
Having a separate partition with no files on it means that there is no fragmentation on the drive.
Better still is if the Drive can run on a separate controller. (ATA or SCSI).
Given the increased levels of Ram that the Mac platform now supports, scratch disks are becoming more irrelevant, as a 200MB file needs 3-5x that in Ram (1GB). Which the Mac may very well have.
