Field efficency is not harvest speed.  The time to harvest the field with a larger head is less than with a smaller head but that is not field efficiency.

Field efficiency is average a/hr divided by running a/hr.  So when you figure a 35ft head at 2.5mph like the OP asked you are picking at 10.6a/hr yet you will not harvest a 106ac field in 10 hrs because you waste time with headlands and turns (efficiency<100%).  Since field efficiency is less than 100%, applying the field efficency to the harvest rate gives for a more realist average estimate.

Since a 35ft head has a field efficiency of 75% the predicted harvest time for a 106a field with a 35ft head at 2.5mph would be 13hr20min at an average rate of 7.95a/hr vs 10hr at 10.6a/hr.  Go down to a 30ft with a 78% efficiency and 2.5mph and you harvest time increases to 14.95hr not 11.66 ideal time.  The wider head takes less time, but field efficiency is lower.