Time to preview will also be influenced by having Highlight/Shadow corrections enabled.
Shooting RAW+ or manually setting raw to one card and jpg to the other will slow things down as well.
Quickest way to view a preview so far has been.
- Card one set to Raw, card 2 set to reserve
- All Lens corrections turned off
- Highlight/Shadow corrections turned off
- Highlight/shadow warnings in the preview turned off
AC Adapter on the way for my 645D ..... looking forward to being battery free in the studio