By the way, more info...
One of the apparently several problems is the Win 8.1 anti-malware software.
Replacing Microsoft's anti-malware package by installing Avast! 9.0 beta now makes networking just as fast as with other versions.
But it still halts, apparently taking some kind of error in the protocol stack, after some files are copied very quickly. Then after a long delay it starts up again.
This is reduced, but not eliminated, by using NAT networking instead of Bridged.
-Noel