Speed depends on three factors (at least):
1. You will always be able to play hands separate faster than hands together. So your maximum speed with hands together will be limited by your speed with hands separate. The most efficient way to increase speed hands together is to increase speed with hands separate. You will find – perhaps to your amazement - that by just working on hands separate your speed with hands together will automatically increase without any hands together practice.
2. Speed cannot be achieved with large movements (speed = distance/time – therefore the larger the movements, the more time it will take you to cover the distances and the slower the speeds). So as speed increases, movements should decrease. This is not to say that you should not exaggerate certain movements when practising. (And some pianists – no matter how famous – have terrible technique).
3. Speed is very much an illusion. The human ear can take just so many sounds. If you play each note clearly, it will sound fast because the ear has to process a lot of information. If you play so fast that either you miss notes, or they become a blur, it doesn’t sound fast at all, it just sounds messy (I can make the computer play pieces at a tempo of crochet = 1000 and faster. It is not a pretty sound, believe me).
Finally, don’t practise giant movements with your fingers. Efficient movement comes from the centre to the periphery, So your fingers should be the ultimate consequence of shoulder/arm/forearm hand movement. Let the arms move the fingers, not the other way around. Alignment of the joints is what you are after. Then even small movements can generate powerful playing. (Think how little your upper arm has to move to displace your hand over half the keyboard).
There are several other threads dealing with similar questions. Check them out.
Best wishes,
Bernhard.
Topic: speed practise