Are you saying that your test is a Go program, or is it the multitest run from above?
In either case, each client process has a single network thread, and it’s possible to saturate that thread. I don’t know if that’s happening to you here, but in order to fully take advantage of a cluster it’s usually necessary to run more than one client process to drive load. To do that, you’ll actually need to start multiple client processes, as we don’t support running multiple network threads in a process.
See also the discussion here: Benchmarking FoundationDB on AWS - #4 by ajbeamon