Individual pixels in the sensor need to receive a certain amount of photons per time unit to record an image, that's why super high speed cameras need super bright lights (the light of a few kW of white LEDs per square meter kind of bright). That is simply how fast a pixel can react, you can't get frame rates faster than the time it takes for the sensor to sense the image. That's also why you have to make sure there's a lot of light inside your piano to have your CCD or CMOS or whatever type sensors pick up the image.
Besides there is indeed also the data transfer rate from the image sensing chip to the outside world, limiting how many images per second you can actually transfer from your sensor to your image capture device. That can be helped greatly by reducing the number of pixels read.