Why are two strokes faster than four strokes?

Both types go through four phases: intake, compression, expansion and exhaust.

A 4-stroke needs two turns of the crankshaft to do this while a 2-stroke can do it in one turn of the crank.

And since it's only the expansion phase that makes power, being able to make power at every turn as opposed to every 2nd turn means a 2-stroke can make more power for the same size engine, or the same amount of power from a smaller engine.

The cost of this is that they tend to pollute more, and have less torque at low rpms.