Watch the Efficiency indicator to monitor performance while you work in Photoshop. Click the pop-up menu at the bottom of the image window and choose Efficiency from the pop-up menu.
If the value in the indicator is below 100%, Photoshop has used all available RAM and is using the scratch disk, which slows performance. If the efficiency is less than 90%, allocate more RAM to Photoshop in Performance preferences. Or, add more RAM to your system.