Slow rsync over Asus Merlin VPN

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I have a pair of Buffalo Linkstations, one in California, one in Alabama. Each is connected via gigabit Ethernet to an Asus router with Merlin WRT software. I've created an OpenVPN link between the routers and devices on one subnet can reach devices on the other. So far, so good. When I use the Buffalo's built-in rsync backup to replicate files from one NAS to the other, the throughput maxes at about 640Kbps (!) ifrom CA to AL, and about 1.2Mbps in the other direction. Available bandwidth according to speedtest is 200Mbps down/20Mbps up from CA, and much better in both directions on the AL side. Round trip ping through the tunnel, NAS to NAS, is 75ms. I have used ACP commander to try to tweak the net stack in the Buffalos to account for the network latency, adding the following to /etc/sysctl.conf and running sysctl -p to invoke the changes:

net.ipv4.tcp_rmem= 10240 192000 1536000
net.ipv4.tcp_wmem= 10240 192000 1536000
net.ipv4.tcp_rmem= 10240 192000 1536000
net.ipv4.tcp_wmem= 10240 192000 1536000
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_timestamps = 1
net.ipv4.tcp_sack = 1
net.ipv4.tcp_no_metrics_save = 1
net.core.netdev_max_backlog = 5000

I'm really at a loss to understand why throughput won't at least reach a couple of megabits per second. Any insight would be greatly appreciated.

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