DVI vs VGA Speed

[JustSo]

Is anyone aware of a speed difference in graphics display when connecting an external monitor via DVI compared to VGA?

I'm using a Toshiba Dynadock U, which supports VGA and DVI connections via my laptop's USB 2.0 port -- my monitor supports both VGA and DVI as well. The VGA connection works reasonably well, but I'm wondering if it would be worthwhile to try a DVI cable, or whether the USB end is the limiting factor anyway.

Thanks!