Last month I ran some tests on an RPI3B+ set up as an IPFire box.
The Green network was the on-board LAN and the Red network was a USB port connected via a Trendnet TU3-ETG (AX88179 USB 3.0 to Gigabit Ethernet).
Edit: updated diagram
The fastest speed from Client #2 to Client #1 was about 105 Mbits/sec
iMac:~ me$ iperf3 -c 192.168.60.3
Connecting to host 192.168.60.3, port 5201
[ 5] local 192.168.1.100 port 56094 connected to 192.168.60.3 port 5201
[ ID] Interval Transfer Bitrate
[ 5] 0.00-1.00 sec 13.3 MBytes 111 Mbits/sec
. . .
[ 5] 9.00-10.00 sec 12.1 MBytes 102 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate
[ 5] 0.00-10.00 sec 125 MBytes 105 Mbits/sec sender
[ 5] 0.00-10.03 sec 124 MBytes 103 Mbits/sec receiver
iperf Done.
and with the -R (run in reverse mode - server sends, client receives) added:
iMac:~ me$ iperf3 -c 192.168.60.3 -R
Connecting to host 192.168.60.3, port 5201
Reverse mode, remote host 192.168.60.3 is sending
[ 5] local 192.168.1.100 port 56102 connected to 192.168.60.3 port 5201
[ ID] Interval Transfer Bitrate
[ 5] 0.00-1.00 sec 18.0 MBytes 151 Mbits/sec
. . .
[ 5] 9.00-10.00 sec 18.5 MBytes 156 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 185 MBytes 155 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 185 MBytes 155 Mbits/sec receiver
iperf Done.