[ubuntu] Samba Transfer Speed Is Slow

Hello.
I have a headless server on Ubuntu 14.04 which runs Plex and nothing else.
When I download a file from this server from a Windows PC, I get a maximum speed of about 7 mbps.
When I download a file using the Plex application (again on the same Windows PC), the speed ramps up to about 30 mbps.
Same server, same PC, and yet such a significant change in speed.

Any idea how I can speed Samba up?

It's hooked in by Etheet using a Broadcom 4401 card.