On another forum post I read that iperf3 needs the -R flag to test download speed, otherwise it's uploading to the server. As per the manpages, it makes sense, so I reran with -R and performance is worse than upload:
$ iperf3 -c openwrt -R
Connecting to host openwrt, port 5201
Reverse mode, remote host openwrt is sending
[ 5] local fdb4:dc8b:aa03:0:19f1:6a8a:1c9c:c191 port 42612 connected to fdb4:dc8b:aa03::1 port 5201
[ ID] Interval Transfer Bitrate
[ 5] 0.00-1.00 sec 14.5 MBytes 122 Mbits/sec
[ 5] 1.00-2.00 sec 15.3 MBytes 128 Mbits/sec
[ 5] 2.00-3.00 sec 14.9 MBytes 125 Mbits/sec
[ 5] 3.00-4.00 sec 14.5 MBytes 122 Mbits/sec
[ 5] 4.00-5.00 sec 15.4 MBytes 129 Mbits/sec
[ 5] 5.00-6.00 sec 15.1 MBytes 127 Mbits/sec
[ 5] 6.00-7.00 sec 15.0 MBytes 126 Mbits/sec
[ 5] 7.00-8.00 sec 15.1 MBytes 127 Mbits/sec
[ 5] 8.00-9.00 sec 16.1 MBytes 135 Mbits/sec
[ 5] 9.00-10.00 sec 16.3 MBytes 136 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.05 sec 153 MBytes 128 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 152 MBytes 128 Mbits/sec receiver
iperf Done.
Closer to the public speed testing services results that I get, which basically peaks at 100Mbps.