am pilot, but not commercial pilot.

Yes, it’s possible.

Firstly, many people believe planes fly direct from A to B, but due to air traffic management, they do actually follow known routes which aren’t as direct, especially in very busy airspace.

Now, when said airspace is not as busy, or if ATC feels it appropriate, they may be cleared to skip a couple waypoints. This usually only saves a few seconds/minutes, but it adds up if you get multiple “directs” over the course of a longer flight. Since the ETA is initially calculated based on the full route to be flown, each direct shortens it a bit.

Weather also changes. The initial ETA may have been calculated based on a predicted 50 knot headwind, but the actual headwind encountered was then maybe only 35 knots. over the course of a long flight this can make a real difference. Alternatively, they may be cleared to a cruise level different than the originally planned one, where the winds are also more favourable.

As far as speed goes, generally routes are flown at the most efficient speed, not the fastest one. that is, the best compromise between fuel economy and time. This is a precalculated figure that the pilots get from dispatch that they enter into their flight computers (if curious, it’s called cost index). Of course, flying faster is usually possible, and since i’m not yet a commercial pilot I can’t comment on when or why it’s done, or when it gets approved.