My other problem with this etude is that when I play the left-hand arpeggio fast alone, it is very smooth. But when combined with right hand I feel less in control, even though right hand chords are easy from a technical point of view. I know the need to practice slowly. And I occasionally alter the speed just to get a different feel, but I know I have to get them very fast for the performance level.
Such problems occur when the hands are acting being isolated from theirs strategic center, or – if they are factually "independent", but in an inappropriate meaning of this word. You feel "less in control" because the speed of your hands' action is greater as a speed of your thinking. The explanation of the next possible aspect of the matter could be the following one: yours each hand uses in action its particular, its "own", different kind of motion, which actually disturbs the co-operation between yours hands! In any case, such complication could happen only because of lack of the effective hearing control. Logically seen, at the first, you have to increase your hearing's ability to work faster; this trick is not possible to implement just playing in the slower tempo, yet! You have to hold your play for a while and check up the real motion INSIDE of the each particular sound. It should happen in your fantasy: you have to generate the vision of a sound that will move – slower, faster, as you like (let you compare this to the Answer 4), but that would not be STATIC. The real legato should firstly mean: you consciously, in yours fantasy, will try to connect a vision of the moving sound to vision of another moving one (moving – to moving, dynamic – to dynamic, not: immobile – to immobile, dead – to dead).
Correspondingly, you should LIBERATE yours fingers and give them even a shadow of possibility to FREELY fall down, using theirs own weight and – when practicing – being slightly accelerated UP to the point, their would "land" at the precisely planned level in the keyboard’s depths in the possibly short time.
Consequently, if playing faster, you should try to realize the same custom: no "bumps", no disturbing "extras"! The moving vision of the sounds in a virtual space of your generous fantasy, in yours hearing, doubtlessly should be positioned on the most honorable place. The hand's action, you, as a whole mind & body entirety – should complete knowing that The Zone must be achieved by the fingers (but NOT crashed) and that the entire virtual action must be started up inside of yours internal space as smoothly as possible. It should be clear that you must be sure, from which artistic reason you like to use just THIS particular kind of the sound's quality at THIS particular place – or another one, maybe...! On the next position stay the fingers: theirs speed and the strictly controlled quality of the sounding results they are able to produce. In the last end NO ONE ELEMENT and NO ONE VIRTUAL FACTOR could be seen as something PASSIVE in playing the piano or any other musical instrument. Amen.
I hope, I do not must explain: this part of our practice that happens in our deep fantasy is as serious, delicate and important thing as one could even imagine; this is a Check Point in the way to the entire art of playing the piano; one cannot pass in using another way.