Being that I run mixed Linux/Win7 networks with SMB filesharing and don't have speed transfer problems, it seems to be a sysadmin problem.
Update your NIC drivers and make sure your processor can handle encrypted sessions at full speed. If you want to downgrade to NTLM you are free to edit the registry keys to do so. The rest of us don't want our passwords and traffic easily crackable.
Thanks for the advice, but the company in question did a lot of work in the networking field, so it is highly unlikely that the poor performance was caused by either a lack of competence on the part of their sysadmins or simple oversights like not updating drivers. Various third party software running on the Windows 7 machines seemed to achieve the expected throughput just fine too, so the finger was definitely pointing to something in Windows being the culprit.
(Really obvious example: An rsync-based tool used to copy files to a central server prior to backing up would do so at roughly the expected speed from all of the Win7 PCs, but dragging and dropping the same files between the same machines in Windows Explorer would take an order of magnitude longer. Just to be clear, I'm talking about testing with new files where rsync would also have to transfer them in their entirety here, not a speed up because of the rsync algorithm itself.)
Update your NIC drivers and make sure your processor can handle encrypted sessions at full speed. If you want to downgrade to NTLM you are free to edit the registry keys to do so. The rest of us don't want our passwords and traffic easily crackable.