I'd never given the automatic handbrake much thought. I always leave Auto Hold on. And there's a final set of traffic lights when leaving the city where I work, which leads directly onto a dual carriageway, so as soon as they go green, if I'm at the front I always floor it like I've sat on a wasp. I've never noticed the brake not releasing fast enough, although I likely wouldn't hear it over my music. There doesn't seem to be any grab so I assume the computer releases it instantly???