Abnormal WIFI speed on MT3000 with VLAN

I have OpenWRT 22.03.5 installed on a computer with 6 2.5gbps ports:

eth0: ISP-A
eth1: ISP-B
eth2: 1gbps switch connecting other devices
eth3: GL-MT3000 (v4.2.3 r1)
eth4: computer
eth5: computer

I want VLAN ID for:
7: LAN (can access everything)
8: IoT
9: Guest

On GL-MT3000, I have removed all interfaces and firewall zones, except the LAN interface.
I have binded eth0.7 (WAN on MT-3000) onto LAN interface, and changed LAN interface to be DHCP rather than static address.

The LAN interface can get an IP addres from the DHCP server on the main router.
There is Internet connection from WIFI.
However, Speedtest.net showed only 1mbps speed.
Having spent a day finding out the problem but no luck.

Finally I have swapped MT3000 with AXT1800 (v4.2.1 r3), the speed is 6xx mbps.
So it seems to be the problem of MT3000?