TWhen When dealing with primitive types like int, the actual operation to push back is not very complex; meaning that a serial process can complete the push back quite quickly. The vector can also likely hold a lot more data before requiring to reallocate its memory. TBB's practicality comes when performing a more complex action on a primitive type or from a simple action on a more complex type. Parallel overhead(the resources required to support tbb in parallel) may also increase the time required which could be the cause of the slowdown in the above comparison.