The basic metric for this kind of comparison is how long can you get your 'compute' node to work on a part of a problem without any new input data and without any intermediate results that need posting for other parts of the code to continue (rendez-vous I believe these are called).

The longer that time the better suited the problem is for a massive parallel solution.

If the time is low relative to the IO that needs to be done then you'll find very soon that the bus that carries data between the host CPU and the number cruncher is the bottle-neck.