It can be. Sometimes you take a profile and there's a big smoking gun and nothing else matters.

Sometimes it's a lot of small things everywhere and you can pick up significant performance after a lot of small value fixes. In this case, caching wire data instead of structured data is almost one of these, because the contribution to response time for serving a cache hit is small... otoh it happens so often than a small improvement matters; but this is a pretty focused use case, you usually hit the many smalln improvement issue in a less focused application where there are many code paths.

Sometimes the whole code structure / data structures are so wrong, but it works and perf is bad and profiling will never tell you. This article is not that case; these data structures only needed refinement.

In many cases, yes. A software pipeline can only achieve as much throughput as its slowest stage, and much of the software we write can be modeled as a sequence of processing stages.