The delay is small (also tested), but it is there. Just that small delay is enough to prevent me to "feel" exactly when the break light is turning on when you start to release the accelerator.I just tested it yesterday sitting in the driveway, with the car in Ready mode but parked.
Whenever I pressed on the brake pedal, CarScanner instantly toggled the setting for the brake light, so no delay.
If the speedometer and "RPM" etc are meant to be real-time, no reason the brake light display shouldn't be.
But I haven't used it out on the road yet - will try to do that later today. I'm hoping it will be useful.