Complex sentence: When an engine was optimized specifically for high speed, it burned perhaps twice as much fuel at that speed than when it was running at subsonic speeds, although the aircraft would be flying as much as four times as fast.
Split into simpler sentences: When an engine was optimized specifically for high speed, it burned perhaps twice as much fuel at that speed than when it was running at subsonic speeds. However, the aircraft would be flying as much as four times as fast.