Is passive diffusion generally fast or slow for most molecules?

Passive diffusion is generally considered to be a relatively slow process for most molecules, particularly those that are larger or polar. While small nonpolar molecules, such as oxygen or carbon dioxide, can diffuse rapidly across the lipid bilayer of cell membranes due to their size and hydrophobic nature, many other substances do not share this property.

Factors such as molecular size, polarity, and charge greatly influence the rate of passive diffusion. Larger and charged molecules struggle to pass through the hydrophobic core of the membrane, leading to a slower rate of diffusion. Therefore, while some small, nonpolar molecules may diffuse quickly, the general tendency for most molecules is to experience slower diffusion rates, validating that passive diffusion is slow overall for many types of substances.