If a human character is wearing fullplate armor and a pair of boots of striding and springing is his speed, taking into account speed above 50ft is considered supernatural for a human.
With the boots, his base speed is 60 feet. 60 foot movement in heavy armor is 40 feet. (Alternatively, huamn speed in fullplate is 20 feet, doubled to 40 feet. This comes up with the same result as long as there's no rounding.)