[ulyssys]

In this example, assuming sequential steps, if step 2 must be performed before step 4, then it is step 2 which is the bottleneck.

After step 2 has been optimized, step 4 becomes the new bottleneck—assuming that optimization of step 2 is satisfactory.

While both steps 2 and 4 contribute to a slow system, a bottleneck means something else entirely: it is the single most significant point of slow down for the rest of the process.

To put it another way, it’s hindering the overall execution. If both use the same amount of time, then whichever is closer to the front of the process is by definition hindering more of the process.