Brian, I have the same experience with the A590 using a class 6 SD card, so I don't think card speed would affect it much. But I haven't really tested it with a slower card.
One thing you might try is using 'click "shoot_full"' instead of 'shoot' in your script. That seems to work faster on my camera. But I think it means EVERYTHING has to be manual because it doesn't wait at all. That would include fixed ISO, and manual white balance, focus, shutter speed and aperture.