[Speed] bm_pickle: why testing protocol 2?

Hi,

performance/bm_pickle.py of the CPython benchmark suite uses the pickle protocol 2 by default. Why not always testing the highest protocol?

In Python 3.5, the highest protocol is 4 which is more efficient than the protocol 2.

Is it a deliberate choice to test exactly the same thing between Python 2 and Python 3?

Victor