The advantages to lower rpm are earlier and more complete fuel burn plus less friction loss. The drawbacks are cylinder leakage and heat loss. The gains improve with lower rpm but so do the losses. However, the losses decline with cylinder size and can be reduced further by eliminating valves. So it is common for large diesel engines where weight isn't an issue to run a much lower rpm and be 2-stroke. This is epitomized by the world's largest ship engine that runs at 15-102rpm and has over 50% thermal efficiency. It weighs 2300 tons so don't plan on putting one in your pickup.
Wartsila Sulzer RTA96-C / Engine The engine is so big that it has staircases built inside the lower part of the engine block to facilitate work on the crankcase bearings.
