Eventually I found that the solution comes from this answer
The answer turned out to be the Large Send Offload (IPv4) setting on my NIC. It was enabled by default. When I disabled it my work upload speed jumped to 12+ Mbps. It appears that if the infrastructure (e.g. routers, switches) don't support this option the transmits become "bursty" and slow down. My home router (an Apple Airport Extreme) apparently does support this option.