If the difficulty and rewards are scaled up more towards raiding (maybe LFR level rewards?), then it'd be perfectly fine for the dungeon to be more than a 10 minute speed run. If you're running the content as your content, and not just ezmode runs for Valor caps then I don't think the GOGOGOGO mentality really needs to be entertained. That can be in the queueable "heroic" dungeon content if people want to run that for VP etc. We don't expect an LFR to take 20 minutes, so I don't think we should expect an additional dungeon difficulty to be speed runs either.
tl;dr stuff.