Do you want me to get technical here? I can get technical... Photoshop is developed by professionals who have support of various hardware manufacturers in graphics and periphrials which allow them to optimize their software for your specific settings. GIMP on the other hand is developed by hobbiest who create ad-hoc fixes and best guess optimizations which tend to slow down the application. Not to mention half of the graphics manipulation routines in GIMP are subpar at best.
GIMP is decent software, no question about it. But it is NOWHERE on the level of Photoshop.