You could check whether
TCP window scaling is working.
Code:
# pfctl -vvss | grep -C1 wscale
all tcp 129.128.5.191:54009 (129.128.5.191:63506) <- 192.168.222.20:38781 FIN_WAIT_2:FIN_WAIT_2
[2816903347 + 102808] wscale 3 [1483341881 + 17376] wscale 3
age 00:00:54, expires in 00:00:41, 239:386 pkts, 12440:575325 bytes, rule 29
See
Create TCP states on the initial SYN packet for an explanation.
Which version of OpenBSD are you using?