Official comment
To expand on @Jay's comment...
The display may not render smoothly depending on your computer's abilities. Try lowering the waveform rendering to MEDIUM in the rekordbox preferences to see if it makes a difference.
As for guest DJs, export USB drives are meant to be connected to devices, not rekordbox computers. Even if a guest DJ brought his rekordbox library on an external drive, he'd then have to load his library, which can not happen simultaneously to having your library loaded. When you drag-and-drop their content into your collection, your computer is analysing the track, which is taking away CPU cycles from the performance, which leads to crackling. Either don't import during playback or increase your buffer to reduce the likelihood of audio dropouts.