Diffusion is the movement of molecules down a concentration gradient from a region of high concentration to a region of low concentration until equilibrium is reached.
The rate (speed) of diffusion depends on:
1) concentration gradient (the more molecules there are, the faster the distribution ; the more crowded the molecules are, they distribute more rapidly).
2) size of the molecules (smaller substances diffuse faster)
3) chemical composition (molecules that can easily dissolve in the bilipid cell membrane can diffuse faster).