I dug a bit more into this - networking was indeed the bottleneck.
Interestingly, I also tried running the Indi server on a RPi3 under Raspbian. It was even slower, with the network bandwidth *looking* like the bottleneck at 11Mbps. However, the whole thing felt sluggish, so I upgraded to a faster MicroSD card. Suddenly images were being downloaded from the server at 37Mbps, a huge improvement. Now the inter-image pause is ~7s, which is quite reasonable.
I've found running wINDI and capturing w/PI seems to flake out after a while and just hang. I am trying now with INDI server running on the rPI to try to determine which side is the source of the flake.