If it was fina a week ago reset your prefs by holding your fingers over Shift, Ctrl, Alt booting Phothsop, and immediately pushing those 3 keys down. You will be asked to reset/delete your prefs.
You may have inadvertedly assign a custom brush to the eraser or have a very large brush. Hard to find out the exact answer, but resetting your prefs should help, if not let us know if anything else is running slow.
I have reset prefs but I still have the same problem....Also I'v noticed that the same problem is with all my programs but more noticeable with PH where you can see the movement best useing the eraser...?
Any other ideas?