The average molecular speed of a gas depends on the kinetic temperature and mass of the molecules that comprise the gas. For a given molecular or molar mass and temperature, the speed is given by the Maxwell-Boltzmann velocity distribution equation. A larger molecular mass results in a slower molecular speed.
A higher temperature leads to a greater molecular speed. The Maxwell-Boltzmann velocity equation allows the calculation of several kinds of speeds for the gas of molecules. The root-mean-square speed, for example, is more effective than calculating the average velocity, because the latter is 0 for randomly moving collections of gas molecules.