Official comment
That's because it's not available within the software when you have hardware control for it:

Crossfader Curve adjustment not working on Rekordbox 4.3.0. in the setting menu. Greyed out for me I have a DDJ-RX. Need help please! Thanks!
That's because it's not available within the software when you have hardware control for it:

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Thanks for the reply! The problem is when my knob on the cross fader curve is all the way to the left and when I move it to A and to the B side it slowly, the music cuts in the middle when I move my cross fader it doesn't transition the sound smoothly from A to B
I have the exact same problem, going from A to B sound cuts in suddenly around the halfway mark. Adjusting the fader curve makes no difference
Hey Colin, I fixed the problem here's the link https://forums.pioneerdj.com/hc/en-us/community/posts/213806023-DDJ-RX-Crossfader-cut-lag-adjust-HOW-
I never seen this reply, thought I would revisit this and got it sorted by adjusting the cut lag. Thanks.
Die Leds lassen sich über den Rücken / Tag Track Regler oben in Mitte ändern.
Nun läuft mein Fader wieder