add Premature optimization

Premature optimization is the root of all evil

In DonaldKnuth's paper "[StructuredProgrammingWithGoToStatements](http://web.archive.org/web/20130731202547/http://pplab.snu.ac.kr/courses/adv_pl05/papers/p261-knuth.pdf)", he wrote: "Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered. We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%."

refer: http://c2.com/cgi/wiki?PrematureOptimization