Final answer:
The correct answer is 'slow.' A mechanism containing a slow initial step implies that the rate-limiting step can have intermediates formed in prior steps of the mechanism.
Step-by-step explanation:
When a mechanism contains a slow initial step, the rate-limiting step may contain intermediates. This is because the rate at which the overall reaction occurs cannot exceed the rate of its slowest step. In a multistep reaction, if the slowest, or rate-determining, step follows a fast reversible step, it may involve intermediates formed earlier in the mechanism. The conceptual framework is similar to traffic flow on a one-lane road where the speed of the traffic is determined by the slowest vehicle. Analogously, in chemical kinetics, the slowest step in the sequence of steps in a reaction mechanism acts as a bottleneck, hence determining the reaction rate. Therefore, intermediates formed in previous steps can accumulate and participate in this rate-limiting step.