It looks like you pretty much found your answer here: http://serverfault.com/questions/366689/how-to-compress-smb
I read other places online talking about putting samba shares on a compressed filesystem, but I doubt that would be advantageous to you. You really do need to monitor the computer and see what the bottleneck is. If I had to take a wild guess not knowing anything about your setup, I'm going to assume you're doing it over wireless and either bandwidth or the router is the limiting factor. If this is the issue, one you guys are closer to the media center you could use an Ethernet cable directly between your machine and the media center. You could also try using multiple routers connected to the server with a switch.
This is a complete guess though. If your machine has low specs, the disk IO could be limiting it, or the CPU usage is topping off. You really have to look at things more closely when you're suffering from the lag.
Heck, you could always just pull a movie from the media center, watch it, then delete it, rather than trying to stream at the same time.