On a fixed-gear bike you are constantly pedaling, you can see this in two ways for speed. Without coasting you have less time for rest, therefore, requiring constant movement to move your bike. A single-speed bike, however, allows for coasting, allowing rest periods to help you go fast when you need to.
A fixed-gear goes exactly the same speed as a geared bike in the same gear ratio. Except for downhill, when your speed is limited by the cadence you can manage because you can’t coast.