I noticed a dramatic slowdown after installing Panther when using Photoshop 7, particularly when using text, layer effects, and also when using the color picker.

After some experimenting I found that changing to the Apple color picker dramatically speeds up text and the color picker comes up instantly. The layer style dialog still takes quite a while to come up, however.

To change to the apple color picker, select Photoshop: Preferences: General and select Color Picker: Apple.