What causes a solid solute to dissolve faster?

Stir it: By stirring the solvent, you expose more solute to fresh solvent, allowing it to dissolve more quickly. It's worth noting that this only maximizes the rate of the combination, and doesn't increase it past its diffusion-controlled limit.

It only goes as fast as the rate at which substances can mix.

Grind the solute: Smaller particles dissolve more quickly than larger ones.