Is low rpms + high load bad for an engine?

[Krush]

Peak thermal effiency almost never occurs at WOT.

Thats where the most power is made. It is most "efficient" at WOT.

"efficient" doesn't mean most power made, it means burning least amount of fuel to make power. Peak effiecinecy usually occurs at the peak in the toque curve.

Friction losses in an engine increase with the square of the RPM...that needs to be considered as well.

BIGASS low speed ship diesels spin at 100rpm and attain almost 50% effiency (meaning near 50% of the energy content of the fuel is turned into mechanical energy)