Poor routing performance compared to masqueraded traffic

Good morning,

I've encountered a strange situation which doesn't make sense to me.
The setup so far is as follows:

  • Banana Pi R4, OpenWRT 24.10.0
  • PPPoE connection to a DSL landline on VLAN 7 in FW zone WAN
  • LAN facing network lan1 in FW zone LAN
  • Default route to the PPPoE connection

What's new now:

  • Another network lan2 connected to a Zyxel NR7302 (in router mode, so holding the public IP and masquerading traffic)
  • Assigned to it's own FW zone 5G, allowing forwarding from LAN zone
  • No masquerading set on the OpenWRT router

Default route is still pointing to the PPPoE device.

I set some specific routes for testing purposes, like some speedtest servers.
Performance downstream is around 150-200 Mbps in this setup.

Now the strange part. If I set the 5G interface to be on the WAN zone, performance reaches up to 750 Mbps. I can monitor that traffic is masqueraded via tcpdump on the port.

I have no idea why traffic should be faster when it's masqueraded.

Any idea how to trouble-shoot?