This animation shows the diffusion of carbon monoxide (CO) into air (a mix of O2 and N2) at two different concentrations. The top panel shows a lower concentration of CO than the bottom panel.
The faster rate of diffusion at the higher concentration (bottom panel) is due to the greater number of CO molecules. The greater number of CO molecules means that there is a higher statistical probability that the CO molecules will spread through the solution, increasing the diffusion rate.
This animation does not take into account other factors, like pressure, that would affect diffusion rates in nature.