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It isn't rpm so much as it is piston speed. Too low a piston speed and efficiency drops because there's too much time for the heat of combustion to escape through the combustion chamber into the coolant. Too fast a piston speed and efficiency drops from increased internal engine friction.
Those super low rpm engines also have long strokes... figure out the piston speed and you will find it to be similar across many engine sizes and types.
Gasoline 4-strokers like 1000-1200 ft/mn. IIRC diesel 2-strokers like 1600 ft/mn.
I have postulated that a heavier flywheel would help me when short-shifting though.
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