The timings on my (4 core) machine are: sequential: 7.7s concurent : 2.1s
Which is about what one would expect. One thing that could be contributing to the timings you are seeing is Turbo Boost (http://en.wikipedia.org/wiki/Intel_Turbo_Boost). Some mobile processors have large (like for example 2.1 GHz vs 3.1 GHz) differences between normal frequencies and maximal Turbo Boost frequencies.
