Why would the simple diffusion of chloride ions across a cell membrane be extremely slow?

There are differnet ways that substances can move across a cell surface; simple diffusion, facilitated diffusion, osmosis or active transport. Most substances can move across the cell membrane via simple diffusion providing they are small and non-polar. However, chloride ions are charged negatively and so they can't cross the membrane down the concentration gradient without any help. This is because polar molecules are water soluble and the centre of the bilayer is hydrophobic. Therefore, they move via facilitated diffusion using carrier proteins.