If you are attempting to copy audio cd's from drive to drive, your max speed most likely is only 8x. This is de to the source cd-rom's limited digital audio extraction rate. Many cd-roms only support 8x audio copying. Try doing the same operation with a data cd, it's likely it will zoom along at 16x, then you can be sure. As some others have suggested, if you were to make an image of the disc, it would burn at 16x, yet you still have to wait for it to make that image.
Hope you figure it out,
Limey