Official comment
The speed of a USB export is entirely dependent of the speed of a USB drive and its capacity to write data from the computer; rekordbox is not the bottleneck. See this video for an explanation.
Cloud sync speed is also dependent on an external system; your home network's upstream speed and the rate at which Dropbox can upload the music to its servers. I have 300Mbit fibre at home and can push 250GB in a day no problem. If you have a large collection and a slow connection, it's like trying to fill an olympic sized swimming pool with a garden hose.
Same applies for downstream if you're having to download the music before exporting it to a USB drive.