The above is a very good idea - you could implement it very easily using two pairs of IR LEDs and phototransistors mounted under each key. Like winner said, when the upper phototransistor switches off, start a timer, when the second switches off - look how long it took.
You would still need to check though if this is actually the right way to go about it - when you watch some people play, it's not so much the speed they hit the keys, but how hard they ultimately push down. Again, back to the impulse thing.
Maybe a load cell of some sort is the best way to go about it - better yet, pull apart an existing one, see how it's done!