For years we've been told that it's our hardware that is too slow for Rekordbox, but even in 2020, on updated hardware and the fastest USB drives, Rekordbox is incredibly slow to do ANYTHING.
It might as well be renamed "Hangtime" as that's what it's best at doing.
You can say all you want about the extra little files and artwork it has to transfer, it still doesn't account for the some of the slowest transfer times known to mankind.
It's nowhere near optimised and never has been. Using it requires you to book out part of your day. Even deleting files is a chore, so much so that you might as well just get another USB when one becomes full.