Depening on how you speedtest, not all parts might be under the influence and control of Proxmox. What speeds do you get when you boot the physical host with an Ubuntu Live instller DVD (without installing)?
What speed do you get when doing a speedtest between two VM or a VM and another system on your local network? Your (available) CPU for virtualizing the network might limit your speed. Did you enable IO Thread for the virtual disks so they won't run on the same CPU thread as the virtual network? Do you select VirtIO for the network?
I expect that people might need a bit more information about your hardware, network and VM to help you figure this out.