Hi, could you take a picture with CHDK night 15 seconds of exposure without having to wait another 15 seconds for the camera to process the image?
Yes - go into the CHDK RAW menu and turn off Dark Frame Subtraction. Even if you are not shooting RAW, it will disable that extra 15 second wait. You might want to capture a dark frame manually though as you will likely need to do some correction on the resulting image when you have dark frame subtraction turned off.