I don't think primers have "burn rates" in the same sense that powders have burn rates. They vary by brissance, the quality and speed of the explosive effect they deliver, flame duration, and flame temperature. I have seen studies that seek to show the effect of different primers on velocity and pressure in a cartridge/gun combination with everything else being equal, but I don't think that any of those studies has conclusively demonstrated one brand or type of primer to be "faster" or "slower" than another.