The speed of a reaction is determined by its rate constant (k), which is specific to each reaction. In a first-order reaction, the rate is directly proportional to the concentration of the reactant, meaning that as the concentration decreases, the reaction rate decreases exponentially. Whether a first-order reaction is faster or slower depends on the specific values of the rate constant and initial concentration. Some first-order reactions may proceed rapidly initially, but as the reactant concentration decreases, the rate diminishes. The comparison of reaction speeds is better evaluated through the specific rate constant and the half-life of the reaction.