One simple way to get better file server performance is to make sure you use a separate server as your file server. In other words, don't use your domain controller as your file server. Why, you might ask. After all, domain controllers don't do all that much most of the time--everyone logs on in the morning, downloads GPOs, and the DC goes quiet, right?
Well, if you are running Windows Server 2003 then SMB signing is turned on by default for security reasons to safeguard network communciations between client computers and DCs by protecting against man in the middle attacks and SMB packet replay attacks. But SMB signing also means that all packets in a TCP session that are exchanged between clients and DCs are serialized i.e. packet 1 must be acknowledged as received before packet 2 is sent, and so on. And this can have a huge impact if you try to transfer a large file between a client and a DC.
Rather than disable SMB signing, which would expose your domain controller to possible attack, why not migrate your file server functions to a separate machine instead.
Cheers, Mitch Tulloch, MVP