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The text linked also has the volume 1 (under 'other editions') with nearly the whole discussion on efficiency. It centers on the efficiency graphs of piston speed vs mean effective pressure. They seem to orbit around the highest efficiency point, but mainly on MEP, not piston speed. So MEP is the key more than speed. At low MEP, lower speeds seem to hurt efficiency.
The Sulzer RTA96C that showed up as the 'most powerful diesel engine' is also quite efficient. It's rated at 92 to 102 rpm, with a 2.5 m stroke, listed 8.5 m/s (1670 ft/min) piston speed but it must be average speed. The highest efficiency is the high speed, lowest horsepower rating. Cross heads don't have much in the way of side pressure problems, but surely they don't have bore size or weight limits either. They must have chosen high piston speeds for efficiency.
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