This animation shows the difference between the diffusion of helium (He, mass = 4)) and carbon dioxide (CO2, mass = 44) into air (a mix of O2 and N2). Carbon dioxide has a mass 11 times greater than helium.
Thermodynamics teaches us that an object with greater mass requires more energy to move at any given speed than an object with less mass (think about the differences between a bowling ball and tennis ball). This is true at the molecular level, as well. Molecules with greater mass (like CO2) will move more slowly and take more time to diffuse at a given temperature than molecules with less mass (like He).
This animation assumes that all other variables are the same between the two systems.