Why move big bitmap is slow?

Hello:
I use VC++6.0 scrollView class .
I display a big bitmap to screen.then I move the bitmap when
mouse move.When the bitmap area smaller than screen area.The speed
is quick.But when the bitmap area is enough bigger than screen area.
The speed is slow.(I must quickly move bitmap.)
Why?Mabey it's scrollbar need setup.But I not know how to do.
Welcome every advice!