I can play Chopin's fantasie impromptu at low tempo, but I'm having trouble when increasing the tempo. My fingers don't move smoothly over 130 bpm and some notes get skipped automatically and sometimes my fingers "hit" the keys. What can I do to remove this defect? Also, give some tips to play Chopin's etude op 10 no 4.
Contrary to popular advice excessive slow practice can be counter productive and lead to "getting stuck" at a ceiling tempo that is too slow.
Once you have the notes under the fingers try practising hands separately at first very quickly in groups of different numbers I.e. In groups of 3 four 5 and 6 and with different accentuation. In very fleeting pieces make sure wherever possible that you keep your fingers close to, and in constant contact with, the keys; this will guard against striking or hitting the keys. Always be conscious of the escapement level at which the hammer goes into 'free-fall'; this is about three-quarters of a centimetre below the top of the keys. in the case of very quick pieces this is in effect your working area. Guard against driving the keys into the keybed using excessive force
You should note that although you should not lift the fingers unduly, the movement through the escapement level should be in acceleration, definite and secure.
Finally, don't over-practice; work in short periods of five to ten minutes of intense attention and then go do something else; carry on like this throughout the day if you have the time. Enjoy it. You've done the background work don't be afraid of coming at it now from a different angle. Good luck