We stop a lot during the day as we ride. For me, the DFR's only take a few more seconds to get into, or out of. Just pull the lap belts across and click it, then snap the sternum strap. For the added security they provide, it's not even an issue. My wife still has a little trouble, but that will go away with more practice.
FWIW, I left the stock belts in place along with my DFR's. I haven't used the stock belt since.