Does iperf have a bandwidth ceiling?

Question

I am trying to run iperf and have a throughput of 1Gig. I'm using UDP so I expect the overhead to pretty much be minimal. Still, I see it capped at 600M despite my attempts.

I have been running:

iperf -c 172.31.1.1 -u -b 500M -l 1100

iperf -c 172.31.1.1 -u -b 1000M -l 1100

iperf -c 172.31.1.1 -u -b 1500M -l 1100

Yet Anything above 600 it seems to hit a limit of about 600. For example, the output for 1000M is:

[  3] Server Report:
[  3]  0.0-10.0 sec   716 MBytes   601 Mbits/sec   0.002 ms 6544/689154 (0.95%)
[  3]  0.0-10.0 sec  1 datagrams received out-of-order

I'm running this on a server with a 10Gig port and even sending it right back to itself, so there should be no interface bottlenecks.

Unsure if I am running up against an iperf limit or if there is another way to get a true 1Gig test.