Among the most important qualities of a rocket engine is its exhaust velocity, which determines its efficiency, delta-V per unit mass of fuel/oxidizer burned.

Among the most important qualities of a rocket engine is its exhaust velocity, which determines its efficiency, delta-V per unit mass of fuel/oxidizer burned. Not exploding is probably the actual most important quality. You don't really start tuning for efficiency until you have got that one nailed. Sometimes absolute thrust is more important than efficiency. Perhaps surprisingly, efficiency can be traded off for thrust by getting heavier molecules into the exhaust. They are coming out slower than the naked protons, but not as many times slower as the ratio of their mass to the protons'.