I've copied mega-gigabytes to and from USB drived many times and I've never understood why the transfer speeds and estimated time to finish fluctuates so much.
1) Transfers can go very fast while the writes are being cached--then slow way down when a cache flush is forced.
2) Flash drives (and SSDs) have a certain amount of space available for writing. Once that's used up pages marked for deletion have to be actually erased--and that's slow. Writing drops to the speed the controller can erase pages. SSDs usually are built with a bigger reserve for writing so this is less of an issue.