A speed booster is any mechanism which allows a signal (indicated by the presence or absence of a spaceship) to move faster than the spaceship could travel through empty space. The original speed booster is based on p30 technology, and is shown below.
Here the top glider is boosted by passing through two inline inverters, emerging 5 cells further along than the unboosted glider at the left.
The fastest speed boosters are the telegraph and p1 telegraph, which can transfer a orthogonal signal at the speed of light, although their bit rate is rather slow.
Diagonal speed boosters have also been built using 2c/3 wires or other stable components. See stable pseudo-Heisenburp.
The star gate seems like it can transfer a signal faster than the speed of light. The illusion is explained in Fast Forward Force Field.