this may an issue with something I complained about a while back and is still not fixed. When you do exports, especially large ones, the export is fragmented REALLY bad and the cdj takes a while to read the contents and database on the drive because of this. I remedy this by doing one of 2 things, defragment the drive (which takes a long time for me) or simply "show hidden files", go into the .PIONEER folder/Rekokordbox and drag the export file to your desktop from the external, delete the export file from the external and empt the recycle bin, then drag back on the export file from the desktop to the exact same location on the external. Believe it or not this makes that one mess of a file defrayed.
I shot a video, it's about 5:00 long but shows you step by step the differences and its pretty substantial. I was hoping this was a big find for the engineers and it is just something in a programming end that can't be fixed to maybe write the pdb file differently or do its own copy/delete/paste at the end of every export. It's a small file so it wouldn't add that much time to the exporting plus it would make a lot of people happy by speeding up the read times when the USB is plugged in but nothing has been done yet. Hope this helps you