What is the difference between low speed engine and high speed engine?

Bore to stroke ratio is a primary consideration. Valve timing (and in the case of a fuel injected engine, the injection timing) will also affect the engine's "power band" rpm range.

For a given displacement, longer stroke = smaller bore dia, more torque and lower rpm.

Diesels are more efficient if they have a lower rpm because the diesel fuel takes longer to burn. So the diesel engine will normally be designed for lower rpm.