Outside of 3-medal situations that need certain speeds, I prefer faster helper squads to zip through the map more quickly. Ideally 600+, but realistically, most fast squads end up being 500-600 depending on what units they have. 700+ isn't really going to happen unless you run Orchid + fast units or if you're lucky enough to have Alpinia (prob my favorite 6 star currently out with her godly speed, HP, lifedrain, and passives).