To reduce the processing time for long exposures, you want to turn off Long Exposure Noise Reduction. However, you may not want to give up the benefit of LENR.

Long Exposure Noise Reduction (LENR) is Canon's nomenclature for in-camera dark frame subtraction. When you take a photo the camera will expose the image normally and then use the same settings to create a dark frame with the shutter left closed. The readings for each pixel in the dark frame will be subtracted from the reading for each pixel in the first frame before sending the raw data to your memory card.

The time required for a dark frame is the same as the time required for the initial exposure - so if you shoot a 30 second exposure you will then have to wait an additional 30 seconds before you can take another shot.

If you disable LENR then you'll find that long exposures will very likely demonstrate more pattern noise (read noise) and hot pixels. You can manually take a dark frame at the end of your session (just shoot a frame at the same settings with the lens cap on and the viewfinder blocked from any stray light). You can then use post processing software to apply the dark frame to your exposed frames.