Wind speed.
The aeroplanes generally travel at a certain speed relative to the air they are moving through.
If the wind is moving in the right direction the plane will go at its normal cruising speed relative to the wind, but faster relative to the ground.
In 2020 there was a big Atlantic storm which led to [the fastest sub-sonic transatlantic crossing](https://www.bbc.co.uk/news/uk-england-london-51433720) when a flight from New York to London did the journey in just under 5 hours. The 260mph winds boosted the plane’s ground speed significantly enough for it to break the previous record.
Obviously if the wind is against the aeroplane it will take longer.
Plus there may be delays at either end due to traffic at the airports; with planes having to circle to wait for a slot to land.
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