The router really does improve upon iperf-only tests.

[Michael Goulish <mg...@redhat.com>]

With large buffers -- like 4 K -- putting the router in between two iperfs
on the 40 Gbit/sec link really does improve upon an iperf-only test.

I confirmed my suspicion/memory -- the reason is, in the iperf-only test,
the receiver goes to 99.7% -- 100.0% CPU. It hits 100% frequently.  And it
is single-threaded, apparently, getting no benefit from multiple cores.

However, wIth the router in between, iperf receiver CPU drops to 85% or so
-- and the overall throughput (as measured at the receiver side) actually
improves noticeably.