My suspicion would be that it's less the speed of the SD card and more the speed of the processor in the camera that is dumping the data to the SD cards. If what's coming out on the SD cards is closer to the original resolution of the image, then the camera would have to do less processing. If it's trying to dump a high res image at low res, then it would have to do more processing and might be using an "optimized" algorithm to get it out, sacrificing quality for speed.

I am, of course, just speculating. :)