Regarding short haul operations, I can think of some routes with overly generous schedules, and others where we consistently struggle to keep to time. Going flat out helps less than you might expect, as the difference between our economy cruise and ‘full speed ahead’ is a whopping 10 knots. On a 1 hour sector, that amounts to a few minutes at best.
What does help are things like expeditious departures when traffic is light (taxy times at Gatwick range from 90 seconds to 35 minutes+) short cuts from ATC, favourable winds (a 100kt tailwind being better than 100kt headwind) and straight in approaches at the other end. Add them all together, and we could easily arrive 15-20 minutes early on a one hour sector.
As Bealzebub says, the schedules are based on an average block to block time, including a ‘factor X’ as a cushion against delays. Occasionally it’s not enough - most of the time it’s more than sufficient, and we’re on stand a few minutes ahead of schedule.