I think many people have seen both sides of this in practice. I’ve seen engineers follow the profiler into a dead-end because they see nothing that stands out in the profiler or they don’t grok the essential nature of the code or system they are profiling. I’ve seen engineers with deep domain expertise consistently make accurate estimates of how a code changes will impact performance without ever using a profiler because their mental model of the code execution maps to reality with high fidelity.
Profilers fill in gaps in our mental model of code execution but they are not a substitute for it. Computer behavior is largely knowable from first principles, albeit requiring a considerable degree of expertise and detail. For some people in some contexts, the profiler adds little information to what they already understand. I know a few people that do almost all of their optimization work using pencil and paper with great effectiveness and precision. Not something I would recommend for most software engineers but not unreasonable at the limit either.
Optimize according to your mental model; profile to check that your mental model matches reality.
If you optimize infrequently, or haven't profiled code like this recently, or haven't profiled this specific codebase recently, then your mental model is probably due a refresh.
In a complex system it isn't always easy to reason about the behavior. Even if you fully understand the cpu, the compiler, the kernel, the disks etc. the way they interact in a real application isn't easy to hold in your head or to deal with using theoretical tools.
I've spent a lot of time optimizing video codecs and while it is maybe possible to reason about these without a profiler it's much faster to profile. The profiler isn't going to tell you what you need to do though.
Profilers fill in gaps in our mental model of code execution but they are not a substitute for it. Computer behavior is largely knowable from first principles, albeit requiring a considerable degree of expertise and detail. For some people in some contexts, the profiler adds little information to what they already understand. I know a few people that do almost all of their optimization work using pencil and paper with great effectiveness and precision. Not something I would recommend for most software engineers but not unreasonable at the limit either.