The accepted answer fails to point out that skidding is less effective at slowing the bike than not skidding. To stop a brakeless fixie quickly, the rider must shift their weight backwards as far as possible (to help prevent loss of traction, as weight moves forwards under braking), while resisting the pedals' rotation as hard as they can, without locking up the rear wheel. Rolling friction > sliding friction.
The heavier the rider, the more difficult it is to stop - there is more momentum to resist. Also, the longer the gearing, the less mechanical advantage your legs have to resist the pedals; again it is more difficult to stop.
However, with manageable gearing (i.e. NOT track ratios!), and good leg muscles and technique, the limiting factor becomes tyre adhesion. Where I live there are hills on which I can only stop with brand-new racing tyres. Worn tyres or harder compounds just give up and skid!