Use mtk sdk HWNAT driver to replace hw flow offload

Hi @MeIsReallyBa
I have managed to compile OpenWrt 22.03 with your drivers for my DIR-878.
It works as expected, but I encountered one problem. I want to separate LAN 1 ports, so I create a new VLAN tagged to cpu and untagged to LAN 1, then I create a new interface for it. The new interface works, I got an IP address and internet connection from LAN 1, but the speed is very slow, around 1-2Mbps. If hnat is disabled using rmmod mtkhnat, I get full speed on LAN 1. Is it normal?
Sorry if this is a stupid question, I'm completely new to this, in fact it took me a week to successfully compile the firmware