Concorde engines were actually some of the most efficient ones while cruising above Mach 1.7 (because afterburners were only used to take off and to go transonic until M1.7). So it was efficient but only when flying fast.

Wiki says: The overall thermal efficiency of the engine in supersonic cruising flight (supercruise) was about 43%, which at the time was the highest figure recorded for any normal thermodynamic machine.

https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593