@Krejzi Try toggle these two and see if that helps:

@Archer_ax20 Correct. Only your LAN wireless traffic can exceed 1 gbps limit.

@Archer_ax20 It's about your local network speed - between your wireless devices @ 5GHz and the AX20. While WAN port determines the max Internet speed, in your LAN you can have faster speeds.

@Inquisit0r Regarding QoS, I asked, because I thought if you have set some kind of a priority, it may causing that, but obviously you didn't. For the following, I have meant in AP mode - "...would...

@Stinger If it needs to be from AX series then refer to this comparison. I would go for AX20.

@agony Try this link. Meanwhile you can try one more thing - disable these two settings (WMM) and reboot the device, then test if that will resolve the problem: