[@Dreaming]
Plasma consists of an ionised gas. In this particular case, a superheated ionised gas. As it is a form of matter, it's speed is dependent upon the forward force applied to it. There is no 'speed of plasma', as it can hypothetically move at any speed. Plasma could be launched at the speed of sound, or at the speed of a turtle. 

Furthermore, your link refers to waves occurring within plasma, not to the movement of actual plasma.

As plasma can move at any speed, and the only reference for it's speed is the game, the only logical assumption is that it moves at a speed approximate to what is seen in the game.