The problem seemed to be with the client resolving localhost.
New (fast) connect URI:
'http://127.0.0.1:50080'
Similarly, adding this line in the hosts file %SystemRoot%\System32\drivers\etc\hosts has essentially the same effect:
127.0.0.1 localhost
Either of these changes increased the speed from 1 call/second to 88 calls/second, and skipping the getfqdn resolution might speed it up slightly more. Not extremely high-capacity, but acceptable for my application.
Correction: the new performance isn't 88 calls/second, but ~1000 calls/second.