For any multi-step process, the length of time that the entire process takes to complete depends upon the time taken for the slowest step to complete. Increasing the rate of any of the other steps will have little effect on the overall process time.
For instance, if you were sitting at the table and wanted to get a drink, the overall process may be described in the following steps with the time taken for each step in brackets:
1. Stand up (0.5 seconds)
2. Walk to sideboared (3 seconds)
3. Get glass out (1.2 seconds)
4. Walk to tap (2.4 seconds)
5. Fill glass with water (12 seconds)
6. Drink water (3 minutes)
The total time for the process is 199.1 seconds. The rate limiting step is step 6. It can be seen that even if you ran between the seat and the sideboard as well as the sideboard and the tap, you would decrease the overall process time by only two or three seconds, which is insignificant compared to three minutes. The best way to decrease the overall process time is to shorten the rate limiting step if possible. Suppose that you did this by guzzling the water in 5 seconds, then step 4 would become the limiting step. In this case, it may not be possible to shorten the rate limiting step if the mains pressure of your area was particularly low, for example, and you have reached a practical limit to how quickly the process can proceed.
This concept is very important to the optimisation and understanding of many chemical processes such as catalysis and combustion
