There are a few components for single tapping, we can split single tapping up into these components:
> The speed at which your fingers taps the key
> The time it takes for your fingers to "respond" by letting go of the key
> The let-go distance your fingers cover
> Your stamina pool
> The leeway between the tapping and letting go of the key
So you want to improve in these criterias:
> Increasing the actual speed of your fingers
> Decreasing the time your fingers is spent having zero momentum, meaning you let go as fast as you can and hit the keys as soon as you can
> Aiming to minimise the distance between your fingers and your keys
> Efficient usage of your stamina as well as stamina growth
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By being more aware of your finger movements until it becomes a habit to take the shortest distance for your fingers to let go, not only would this help improve your speed but also your stamina and your control. If you have more control of your finger movements than you'll probably be a lot more consistent with BPM and you're not wasting energy with needless movements
Having your fingers react a lot faster to the up and down movements will also increase your single tapping capability, as a fast up/down motion is useless if you aren't actually starting that motion as fast as you can. Faster finger movements and slow reaction time inbetween each tap will wear you out a lot faster than moderate finger movements and fast reaction time inbetween each tap.
The biggest factor to any of this is your stamina pool. If your stamina pool is crap then there really isn't any other way to "get better" at streaming until you actually have something to work with. Increasing your stamina pool should be common sense; not anyone with a good stamina pool is good at streaming or tapping or hitting keys, it's how your stamina is allocated and making the most of it. For example, a player with a huge stamina pool could be totally ass at tapping, because his tapping technique consists of a finger motion distance that is double of a mediocre player, so the tapping speed of his motions use a lot more force than the mediocre player.