I have been facing the same issues too with my RB USB drives & also using SanDisk 128GB Ultra & RX3s.
I *think* it could be different factors: The USB being a large size & RB tracks. Always reading/writing/removing tracks from the USB drive, plugging in & ejecting the drive from the CDJ.
I think it could be general wear-and-tear... But having said that, my most newest USB is same brand/model, only bought a few months back and was ejecting rather quickly when i started using it but now is facing same slow ejecting issues on CDJs
Anyways, i hope Pioneer can shed some light onto the possible issue