For years I have been battling a continual problem where Samba file transfers to my mac would run like absolute crap. Horribly slow speeds, in the 30Kps range, rendered samba completely useless.

A google search today finally turned up a solution that worked PERFECTLY!!

sudo sysctl -w net.inet.tcp.delayed_ack=0

Running this in the terminal fixed it completely, I now have transfers at full speed.

This article was my savior.