This is actually untrue in a great many cases, especially where very rapid passagework is involved. If you film a (skilled) pianist playing a rapid passage at speed, and then film the same passage played slowly (as if it were *intended* to be played slowly) and then play the first film back at the speed of the second, you will see something very different. There is a way of practicing slowly music intended to be played rapidly, but it involves some difficult adjustments of weight and motion. I'll give a simple example. In scales of slow to moderate speed, there is time for the thumb to pass under the hand well in advance of having to play. In extremely rapid scales the thumb cannot pass under the hand.advance and in fact such scales are not physically legato. This is a reason why players get 'stuck' when trying to hit a certain speed - they are hitting the limits at which certain motions can be performed, not realizing that they are the wrong motions.
I recommend reading Otto Ortmann's 'Physiological Mechanics of Piano Technique', if you can find a copy. Ortmann was the first to describe this issue precisely (as opposed to skilled pianists, who just 'do' it
Marvin Wolfthal