Most cameras are USB 1.1 - the card readers are MUCH faster. Many newer DSLR cameras shoot to SD cards now and transferring data over the USb cable can be awfully slow.
The card slot isn't a replacement for a FW800 card reader with UDMA cards but it's a damned sight better than the slow cable connection from the camera.
I haven't noticed slowness; then again, I don't stare at the screen mouth breathing whilst the transfer is taking place - I go and get a coffee, pull a cone or take a leak.