For some more info on what I wrote earlier, speed is limited at altitude by Mach buffet, which depends on the design of the specific airplane. As altitude increases, the low speed stall speed increases which eventually creates a ‘coffin corner’. The window between the high speed and low speed limits can become quite narrow (depends on weight and altitude), and the speed that is flown needs some margin on either side. Turbulence causes greater speed fluctuations so can require a larger speed window to operate in.
Normal cruise speed on a 737 is Mach .77 or .78, limit is .82 (in smooth air). If you want to speed up by going from .78 to .80, as long as it’s fairly smooth, it only increases speed a little over 2%. So on a 5 hour flight, it would save less than 10 minutes. But fuel burn would increase significantly more than 2% (I don’t have specific numbers handy).
While there is some ability to bias between speed and fuel economy, it isn’t something dramatic like 20-30% different in speed.
If a flight is significantly under or over block (scheduled flight time) it is likely because of ground congestion (taxi times less or more than typically expected) or winds different than usual. If a flight is planned based on 50 knot headwinds, but today there are 50 knot tailwinds, we suddenly are doing a ground speed (which is what shows up on flight tracking sites) that is 100 knots faster than planned. Thats a 20~% increase and would save significant time.
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