SamyGO

It's source on linux kernel. You can download sources from samsung's page.
But I think your problem is not CPU bottleneck perhaps WiFi bottleneck.
I don't think if you can optimize cifs sources. But you can try...
You will also need SamyGO OpenEmbedded for building tool chain to compile cifs.ko.

Yeah, that would be so great to optimize cifs.ko. I only got a about 2.5 MB/s via cifs.ko in a N WLAN which is not enough for some 1080p contents.