I don’t see your logic. It should be more efficient to unload the same number of containers on fewer ships because there’s less time spent moving the ships into and out of position at the dock.
If your infrastructure is set up to unload X number of containers a day, and now you're dealing with ships carrying twice as many or three times as many containers then your unloading time doubles or triples. You may have been able to process seven ships per week. Now you can process only two or three.