The rate of simple diffusion for a solute depends primarily on

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Membrane permeability to the solute sets how fast it can cross the phospholipid bilayer without help. The diffusion rate across a membrane hinges on how easily the solute partitions into and traverses the lipid bilayer. Small, nonpolar molecules pass readily, giving a fast rate, while polar or charged solutes have lower permeability and diffuse much more slowly unless aided by channels or carriers. Temperature can alter diffusion speed by changing molecular motion and membrane fluidity, but it mainly modulates the overall rate rather than defining the baseline permeability for a specific solute. The size of the cell doesn’t change how quickly a particular solute crosses its membrane, and water solubility isn’t the key factor for crossing the lipid bilayer. So, the primary determinant of the rate of simple diffusion for a solute is how permeable the membrane is to that solute.