Concurrency and program speed

[jerro]

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.