The crushed pill has a larger surface area (contact between the solid tablet material and the water). The greater the surface area, the faster the dissolving (and the faster the reaction as the pill dissolves). The concentration of the materials in the Alka-Seltzer cannot be changed, but you could add more than on tablet to change the concentration. For this reaction, the higher the temperature (until the water boils), the faster the reaction. Catalysts usually increase the rate of the reaction. I do not know of any catalyst that you could use for this reaction.