TimeTBB slower with more threads

[mfinean]

Running the example TimeTBB I get the following results (faster using 1 thread). I'm currently looking into why but any insight in the meantime would be greatly appreciated.

numberOfProblems = 1000000
problemSize = 4
With 1 threads:
Without memory allocation, grain size = 1, time = 0.195355
Without memory allocation, grain size = 10, time = 0.194243
Without memory allocation, grain size = 100, time = 0.194409
Without memory allocation, grain size = 1000, time = 0.196936
With memory allocation, grain size = 1, time = 0.289234
With memory allocation, grain size = 10, time = 0.295508
With memory allocation, grain size = 100, time = 0.294618
With memory allocation, grain size = 1000, time = 0.29145

With 4 threads:
Without memory allocation, grain size = 1, time = 5.02581
Without memory allocation, grain size = 10, time = 4.9835
Without memory allocation, grain size = 100, time = 4.74276
Without memory allocation, grain size = 1000, time = 5.06713
With memory allocation, grain size = 1, time = 4.6808
With memory allocation, grain size = 10, time = 4.73614
With memory allocation, grain size = 100, time = 4.77174
With memory allocation, grain size = 1000, time = 4.75051

With 8 threads:
Without memory allocation, grain size = 1, time = 4.00496
Without memory allocation, grain size = 10, time = 4.06559
Without memory allocation, grain size = 100, time = 4.06971
Without memory allocation, grain size = 1000, time = 4.06233
With memory allocation, grain size = 1, time = 4.65368
With memory allocation, grain size = 10, time = 4.6617
With memory allocation, grain size = 100, time = 4.65855
With memory allocation, grain size = 1000, time = 4.65979

Summary of results:
4 threads, without allocation, grain size = 1, speedup = 0.0388704
4 threads, without allocation, grain size = 10, speedup = 0.0389772
4 threads, without allocation, grain size = 100, speedup = 0.0409907
4 threads, without allocation, grain size = 1000, speedup = 0.0388654
4 threads, with allocation, grain size = 1, speedup = 0.0617917
4 threads, with allocation, grain size = 10, speedup = 0.0623943
4 threads, with allocation, grain size = 100, speedup = 0.0617423
4 threads, with allocation, grain size = 1000, speedup = 0.0613514
8 threads, without allocation, grain size = 1, speedup = 0.0487782
8 threads, without allocation, grain size = 10, speedup = 0.0477773
8 threads, without allocation, grain size = 100, speedup = 0.0477697
8 threads, without allocation, grain size = 1000, speedup = 0.0484786
8 threads, with allocation, grain size = 1, speedup = 0.0621517
8 threads, with allocation, grain size = 10, speedup = 0.0633907
8 threads, with allocation, grain size = 100, speedup = 0.0632425
8 threads, with allocation, grain size = 1000, speedup = 0.0625457