We found out that propane has like 15,000 btu’s per pound and gas has 114,000 btu’s per gallon ... so we stayed with gas.
Don’t compare pound to gallons, that doesn’t have meaning.
But yes, if you take an engine designed for gasoline and feed it fuel through the same delivery system with less energy per volume, it is not going to perform as well.
But these engines are designed to use the Nat Gas.