What is the latency between the two sites? That is going to dictate how much potential throughput a given TCP/IP stream could use. You may need tune your TCP/IP window to increase the throughput.
I would check the network to ensure there are no errors occurring; i.e. dropped packets and retransmits.
Do you have QOS enabled and if so how is that handled? Does it just drop the packets? As that would cause the TCP/IP to backoff and retransmit.