DCRAW is used to read the data from the CR2. For frame and focus images you can use the "JPG" option in the frame and focus module to speed things up. That will pull the JPG for display and it will be in color (debayering technically happens via the camera as JPG is stored in the CR2 file).
I'll move this to the feature requests for us to look into the transfer speeds a little more. I can't promise if/when that will happen. Only that we'll look into it and see if we can speed things up.
Thanks,
Jared