memory save speed

I'm not sure if this is a general thing that happens with the camera or the memory card. When the exposure time is long it takes just as long to save the picture eg. if the exposure time is 20 seconds it take another 20 seconds to save it before i can take another picture. Why is this?
cheers

Something to do with noise reduction (dark frame subtraction) .. you can turn it off on the K10D, not on the K20D and for the first 30 seconds on the K-7.

got it, "slow shutter speed noise reduction" (feel kinds stupid now that i think about it ), how does that affect the quality of the pictures?