I think Windows just sucks and they haven't changed how file copying is done since DOS 2.1. When I use Teracopy, it copies those large files quicker, and more consistently without the Bipolar Progress Bar!
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.