I was a passenger on an east to west flight recently. During the flight I was playing with Garmin Pilot's new "Flight profile" feature that nicely shows the wind speeds at various altitudes, and I noticed that we were flying against an 80+ knot headwind at FL320, but if we'd dropped to FL220 we'd shave about 40 knots of headwind. That seems pretty significant. A 1,000 NM flight with 355 knot ground speed (which we were cruising at) would save 17 minutes of cruise time with 40 knots less headwind. That's a 10% increase in efficiency.
So the question is, did we stay (or even plan in the first place) to fly at FL 320 the entire cruise portion of the flight simply due to ATC needing to keep us there for traffic routing purposes, Is there an even larger increase in fuel efficiency between FL320 and FL220 than 10%, or is the difference negligible enough that it's not worth asking for a different altitude from ATC, or some other factor not considered here?