I'm have a quite peculiar and highly frustrating problem.
I have recently set up a Windows Server 2012 file host, sitting on top of kvm, using virtio drivers and a PCI-E passthrough HBA (debian host).
Performance is great internally, and I get great download speeds from the network shares (essentially full disk/network speed).
However I have found that the two Windows 8 Pro desktops that I have connected to the domain (separate Windows 2012 server on the same KVM) effectively cannot upload files to the server.
They either immediately drop their speed to 1kb/s or spend forever "calculating". I tested uploading from a Windows 7 VM on my MBP and the upload speed if perfect.
I'm beginning to think that something isn't right with SMB 3.0? (as that would only effect the Windows 8/Windows Server 2012 combination). I have tried playing with a few of the network adapter settings (disabling offloads/checksums) with no difference.
Network adapter drivers are all up-to-date on both server and client.
It looks like it's going to be a setting on the server somewhere as I'm happy with the speeds between two Windows 8 Pro machines (which will also be using SMB3.0)