Rekordbox performance mode with cdj 2000nxs & 900nxs crackling audio and slow response issue

Hi I've connected my pioneer 2000nxs and 900nxs to rekordbox with usb cables and audio through a external mixer, the sound is either choppy, crackling or slow stuttering and the jog pads aren't very responsive with the program, I connected them via the menu on the cdj and set the deck number to correspond with the two in rekordbox, any suggestions on solving this issue would be great thankyou