A cargo cult wisdom snippet is the belief that compilers automagically apply every possible optimization. But they don't, because compilers don't know everything you know. Compilers have to assume that your code can be subject to side effects you never thought of, that would change the result of the code, so a lot of possible optimizations are avoided.

Well you check the compiler output and profile of course. But "generalized" performance advice like "use reverse loops" hasn't been relevant for a long time, it's a similar myth like "pre-increment is faster than post-increment".