It depends on the winds, in some months the jetstream tailwinds can be wild, the eastbound aircraft can arrive an hour early at destination. Pootling along with a slower airspeed just slows everyone else done which in busy congestedd airspace doesn’t help. The other option of blocking a gate for an hour or sitting on a remote stand while staff wait around is also not ideal. They can come back on the speed once in controlled airspace beyond the oceanic area, but there are a whole load of variables. It’s genuinely not as simple as there’s a lot of other slots and airspace permissions to take into account as well as departing too late may mean the meal service is horribly out of sync with body clocks.