And the varying file copying speeds tell me that something brain dead is going on.
Copying a single large file brings transfer speeds up to 50MB/s (400Mbit/s). Copying lots of small files brings this down to 1MB/s, and occasionally down to 300kB/s when dealing with multiple tiny files.
So either there's a massive overhead in setup/teardown of individual network connections for each transfer (which would be dumb), there's a massive overhead of updating the file-system for each file (which would be dumb) or at the end of each file it's pausing and flushing all the buffers before going on to the next one (which would be dumb).
In any case, there's _something_ dumb going on.
Original post on Dreamwidth - there are comments there.