Hugh,

Jet engines really do become less efficient at partial power settings.

At its T/O power setting, a modern turbofan uses about 0.3 pounds of fuel, for every pound of thrust, per hour.
If it is making a maximum of 20,000 lbs static thrust, it will use 6,000 lbs of fuel per hour doing it.

At a cruise setting, at 36,000 ft, making about 25% of its rated T/O thrust, at 0.8 Mach, it will be using about 0.55 pounds of fuel, for every pound of thrust, per hour. At 5,000 lbs thrust, it will use 2,750 lbs of fuel, per hour.
At this altitude, the thin air allows this 25% power cruise setting to give the plane a very high cruise speed.
Often it is over 90% of the plane's maximum speed.

This would seem the engine is a little more than half as efficient at cruise power, compared to T/O power, but actually it is different, because the cruise thrust is rated is at cruise speed, it is not static.
A turbofan is part jet, part ducted fan. Its thrust is reduced as speed is increased, somewhat like a propeller's.
Its output is rated in thrust, when actually, in part, it is has "horsepower" characteristics.