how to coordinate hands when playing very fast

The below image is measure 57 from Chopin, Nocturne in C sharp minor, and the time signature is 4/4.

I started by playing it slowly, then sped up. When I become fast enough, my hand loses control over the keys, and I can't match the left hand with the right hand. Is there a better technique I can use to become faster and have control over the keys?

The most effective way I've found for passage like this is to break the faster right-hand notes into groups and match them to the left hand. Then resume slow practice, but with both hands together, and gradually speed up.

For this particular Chopin passage, I recommend breaking up the right-hand this way:
`3+4+5+6` or `4+4+5+5`.

It would be notated this way:

``````X: 1
T: Nocturne in C# Minor
T: m. 57, beats 3-4
C: Chopin
M: None
K: C# minor
L: 1/16
V:V1
V:V2 clef=bass
%%score (V1 | V2)
[V:V1] (3def g/2a/2b/2c'/2 (5:4:5d'/2c'/2b/2a/2g/2 (6:4:6f/2e/2d/2c/2B/2A/2 |
[V:V2] C,2F,2D2C2 |
``````

or

``````X: 0
T: Nocturne in C# Minor
T: m. 57, beats 3-4
C: Chopin
M: none
K: C# minor
L: 1/16
V:V1
V:V2 clef=bass
%%score (V1 | V2)
[V:V1] d/2e/2f/2g/2 a/2b/2c'/2d'/2 (5:4:5c'/2b/2a/2g/2f/2 (5:4:5e/2d/2c/2B/2A/2 |
[V:V2] C,2F,2D2C2 |
``````

Also, it is stylistically acceptable (even expected) to "stretch" the time in this passage, slowing down slightly for dramatic effect. This has the benefit of making the fast run easier.

• Pardon the weird barline in the first example. ABCjs rendering bug. Commented Dec 25, 2020 at 7:25

In such a case I find various editions with precise complete fingerings of a same song/piece and search and play relevant preparatory exercises from Brahms, Liszt, Clementi, Godowski, Jonas. When playing rhapsodic passage against a bass line you can find corresponding points of notes or learn the bass line first precisely with metronome and thereafter put your right hand freely from slow to fast. Left hand is a strict conductor, right is a free voice (From F Chopin letters..)