While conducting tests of different browsers using SunSpider Benchmark, the engineers at Mozilla found a rather queer bug. IE9 was specifically altered to load an optimized result while running a benchmarking tool. Mozilla’s engineer Rob Sayre reported his findings with Microsoft in Microsoft Connect.
The trick lies in the fact that, IE9 performs really well in an unaltered SunSpider Benchmark, but when benchmark is altered by adding more of the dead codes, it doesn’t perform as expected.
It is being speculated, that IE in itself is not performing these optimization on a real world scenario but rather has been optimized to give a good result in benchmark.
A hot debate is going on in Hacker News and many have performed independent benchmarking tests to come to the same conclusion.