Computational Fluid Dynamics: 1 Core VS 2 Cores VS 3 Cores VS 4 Cores. (SolidWorks Flow Simulation Premium)

Problem:
Lift and Drag calculation for a rectangular wing (0.5 m span x 0.15 m chord x uniform cross section of NACA 9410), at horizontal velocity of 10 km/h.

A very simple CFD problem (with 177250 cells) solved on a 4 core Haswell-DT, varying the number of CPU cores used each time; CPU times are as below:

Cores used            CPU time taken for solution (1 travel convergence)
1/4 core                 782 seconds (2.31x slower)
2/4 cores               468 seconds (1.38x slower)
3/4 cores               380 seconds (1.12x slower)
4/4 cores               339 seconds (1x)