I had a case with the Zyxel support with nearly the same Problem which came up when a VPN is configured.
I am very unsatisfied with the answer, because when you are on a different lan you get other Problems and have to add routing and so on!
Answer from Zyxel support:
In this case, USG60W with slow speed performance is because the Bridge problem. In current ZLD design, default bridged WLAN SSID to Ethernet LAN1, when there’s traffic packet transmit will into Bridges checking bridge table packet and decide to WLAN or Ethernet LAN. This will decrease and effects on throughput as well.
Therefore, base on our design,
to get better performance, user need to change the setting on local VAP Setting from Lan1 to Lan2.