Problem was Large Send Offload (IPv4) setting on my NIC. It was enabled by default. When I disabled it my upload speed jumped to 11+ Mbps. It appears that if the infrastructure (e.g. routers, switches) don't support this option the transmits become "bursty" and slow down.
To disable, go to Start->Control Panel->Device Manager->Network adapters->Atheros Gigabit Ethernet Controller (Whatever Brand NIC card you have), right-click then select Properties. On the Advance tab, find property Large Send Offload (IPv4) and change its value to Disabled then click OK.