@mikey_s I haven’t had the opportunity to try it yet but per https://docs.netgate.com/pfsense/en/latest/vpn/openvpn/dco.html:
“Thus, DCO is beneficial even when only one endpoint is capable of DCO. That said, tunnels employing DCO on all peers will see the most benefit. With DCO on only one peer the performance improvement can still be notable but not as significant as the gains with DCO support on both endpoints.”
That page also says it’s (still) experimental.