10G NAT/Firewall performance problems

[farmwald]

I have a 10Gbps fiber connection. Currently, I use a UBNT EdgeRouter Infinity router/firewall, which works quite well. I get 4Gbps down, 5.5 Gbps up with speedtest, which seems about right.
I'm testing other firewalls, specifically OpenWRT, OPNSense, and PFSense on some medium to good performance PCs (i3-8100 and i7-7xxx) with 32GB of memory.
For straight routing (no firewall no NAT), all of the firewalls give me well over 6 Gbps - probably limited by the test rather than the router - on all of the PCs I've tested. This is fine.
When I use Speedtest (i.e., using NAT and a firewall), I get very odd results:
OpenWRT 18.06.2 ~100 Mbps down, > 4 Gbps up
OPNSense 19.1 (and 18.7) - ~100 Mbps down, > 4 Gbps up
PfSense CD 2.4.4r1 - 4 Gbps down, > 4 Gbps up
These are fresh installs, with no tweaks or tuning. The numbers are consistent across different PCs. I'm using an Intel X540-T2 for all tests and I've tested 3 PCs with every firewall, so it's not hardware dependent.
Clearly, there is a problem with my use of OPNSense and OpenWRT, but I don't what to try to fix this.
Why does PFSense work out-of-the-box, and nothing else?
Any suggestions?
 

[mimugmail]

Try to set MSS on LAN to 1300

[farmwald]

Try to set MSS on LAN to 1300

That fixed it. 25x faster from Speedtest.
Thanks.