Optimisation effort Vs. Performance Gain

This may seem strange but in my experience it is true that the first few optimisation efforts on any un-optimised system tend to yeild high returns for low effort e.g, tweaking some compiler or linker settings in a few minutes.  Further medium term efforts tend to suffer from diminishing returns e.g, rewriting critical sections in assembler over a few days for a few percent improvment.  Finally large scale optimisations e.g. refactoring entire sub-systems over a week or more tends to yeild significant benefits.

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