I'm aware of Amdahl's law. I also know of a case where adding an index to a production database gave a factor of 1000 improvement. If your design started as a proof-of-concept, you made a rash assumption, you had a tight deadline or you're just plain incompetent then there's almost no lower bound to initial system efficiency.
My comment was aimed at your down-modded comment. You can make 10% performance improvements in dozens of modules throughout your system, but if they only account for, say, 5% of the total execution time, it won't make much difference.
