FWIW, shutter speed is only relevant in low light conditions. If you are talking about a ~1fps shooting rate, +/- 1/60th really isn't worth worrying about.
If using RAW mode, then disable noise reduction in CHDK RAW menu (if using long exposures)
I'm guessing you are talking about "dark frame subtraction". Despite being in the raw menu, this setting is applied whether or not you are shooting raw.
If left in auto, the camera will generally apply dark frame subtraction if the exposure is longer than about 3 seconds, although this may depend on the model and sensor temperature.
In the Low Light mode, the rate increases to 3 shots per second.
On many recent cameras, this can be called "High ISO" or "ISO 3200" etc. Using CHDK, you can force the ISO lower while still using these modes.