what has gas velocity got to do with it? is it thought that the gas velocity is what drives the turbine at speed?

surely it is the volume flow of air through the turbine that determines how fast it turns? obviously if you have the same volume of air flowing through a larger turbine, the gas speed will be slower, but does that mean that the turbine will spin slower? is it not related to the pressure differential either side of the turbine?

i hope to learn something from this post, so thanks alex