The schedules are padded a bit and that helps.

Most airlines have pilots flying FMS equipped airliners at low cost indexes which yield generally lower speeds. A typical cost index of 35 (out of 999) will yield a mach number of between .76 and .79 depending upon winds, altitudes and most importantly length of the flight.

Most narrowbodies have a maximum speed of around .82 mach while widebodies may fly faster. (the 747 can cruise at .86M)

The longer the distance the more affect a faster climb and cruise speed will have on arrival time.

Going faster on a five hundred mile segment will have very little effect on flight time. It is not worth it to burn 2000lbs of fuel to arrive five minutes sooner.

The last time I flew significantly faster than planned to make up time was on a flight from PHL to SAN. Push was an hour and twenty late but we arrived on time. There were several factors in play. There was padding built into the schedule to cover both delays on the ground (not unusual in PHL) and the general slop in the schedule.

Twenty to thirty minutes was saved simply because we were lucky and had minimal taxi delays on the delayed departure.

The cost index called for a climb/cruise at 280Kts/.77M. I flew at 340/.82 and was able to cut forty minutes off of the flight time. Had the segment been any shorter the faster flight speeds would have had a negligible affect.