Most burners are slower on CD-RW discs than CD-R discs; e.g. usually a burner that can write CD-R at 8x can do RW at 4x. It probably has to do with the laser power required to burn at a certain speed, perhaps CD-RW requires twice the power for a given speed as CD-R. But then that's just a wild guess on my part.
I noticed when perusing Dell's site that none of the drives can write RW discs at more than 10X, regardless of their CD-R writing speed. But for slower drives (8X CD-R) it's usually twice as slow for RW (4x).