The Colibri PXA3xx come with a Graphics Controller Unit (GCU) for 2D graphic acceleration.
In many cases the CPU can do the same operation faster. But the good thing is, when the GCU is working on an operation the CPU can do something else at the same time.
If you use just the normal GDI functions, in the background the display drivers tries to use the GCU when ever possible. You can change this behaviour in the registry.
Check the "Blit" Key in the Display Driver Registry Settings.
The display driver can only use the GCU if the graphics are in the VideoRAM.