^^That^^ It depends on the size of the files being transferred, more specifically lots of small'ish files will hit an I/O, Windows, or file system limit before a bandwidth limit.
IIRC Windows built in file copy uses a single thread for file transfers so you can hit a CPU bottleneck, then there's having to write data to the journal (a log of the filesystem changes), and finally if you're transferring to a HDD or USB you can be bottlenecked by the input queue getting filled up (low IOPS).