When PF firewal runs local network traffic is slowed

OS FreeBSD-13.0p3

We have a gateway router (G) with three physical Ethernet interfaces. One (W1) is the WAN gateway. The other two (L1,L2) are connected to the same wire. L1 belongs to the 192.168.0.0/16 network. L2 belongs to our public routable network 123.123.123.0/25.

All single-homed internal hosts belong to one or the other networks exclusively. Some dual-homed hosts have one nic on L1 and the other on L2.

We use PF firewall on G.

If PF is not running then hosts on L2 can immediately connect to hosts on L1 and vice versa. If PF is running then there is a significant reduction in bandwidth which manifests itself with extremely long logon times and slow file transfers.

I was told that because L1 and L2 are on the same system that the OS network stack should route traffic between the two without the firewall being involved. But this seems not the case.

I realize that this is minimal information, however, in the event that someone recognizes the symptoms I am posting this here.

An explanation of what is going on is much appreciated.