Performance issue while copying data from video memory to system memory

Hi,

Right now i am developing one driver. In the driver i am able to get
framebuffer pointer. When i use frame buffer pointer to copy data from video
memory to system memory it takes around 200ms. I am transfering 3 MB of
data. Then i tried transfering same amount of data from system memory to
system memory and it takes around 15 ms.

Can anyone explain me the reason? And it will be really helpful if anyone
can help me improve performance while copying data from video memory to
system memory.

Thanks
Manish