Works fine on my ER-X running v1.9.0:
ubnt@ER-X-1:~$ iperf3 -c 172.16.3.15 -u -b 4M Connecting to host 172.16.3.15, port 5201 [ 4] local 172.16.3.23 port 41500 connected to 172.16.3.15 port 5201 [ ID] Interval Transfer Bandwidth Total Datagrams [ 4] 0.00-1.00 sec 464 KBytes 3.80 Mbits/sec 58 [ 4] 1.00-2.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 2.00-3.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 3.00-4.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 4.00-5.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 5.00-6.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 6.00-7.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 7.00-8.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 8.00-9.00 sec 512 KBytes 4.19 Mbits/sec 64 [ 4] 9.00-10.00 sec 512 KBytes 4.19 Mbits/sec 64 - - - - - - - - - - - - - - - - - - - - - - - - - [ ID] Interval Transfer Bandwidth Jitter Lost/Total Datagrams [ 4] 0.00-10.00 sec 4.95 MBytes 4.15 Mbits/sec 0.056 ms 0/634 (0%) [ 4] Sent 634 datagrams
Note: testing with iperf on the router is useful, but doesn't really test how fast the router can forward taffic because it's generating packets not forwarding them.