Hi!
On my A630, there is an option, like a digital zoom, or crop, which allows me to shoot at 3 or 2 mpix instead of 8, and gives me a crop factor of 1,6 and 2 respectively. And as the whole surface of the sensor is not used, I noticed that the burst mode is a little faster also. I don't know if you camera has this "crop" option (I think Canon calls it "safety zoom") , and if so, do not forget to change the resolution to either 3 or 2 mpix, according to the crop selected, otherwise, the camera will have to upsample a smaller picture, resulting in a loss of time again.