How fast does a jet fly?

They have been flying jets relentlessly down the Lake District valleys recently. There's one in this picture against the lowest cloud right of centre. We were on Silver Howe above Grasmere and I timed it fly from roughly White Moss Common to roughly Dunmail to try to work out the speed. It took 20 seconds. The distance is maybe 3.5 miles. That's 630 mph. The speed of sound is 767 mph. They mustn't go faster than sound because the sonic boom would break windows.