Wow. Thanks everyone. I was using the default options for iperf3. Using iperf3 with the -w 1M option fixed it, and I am measuring >900 Mbps. This works both with the included 3 ft Coax cable or using my home's in-wall wiring (much longer than 3 ft).
I have two Actiontec ECB6200 adapters, and I am testing performance using iperf3 between two Windows 10 PCs. The highest performance that I've been able to measure using the Moca adapters is around ~380 Mbps between the PCs, which is great but slower than I was expecting. I can get >850 Mbps...