In the Ubiquiti U6-LR Uplink activity moniter is turned on?
The GL-AXT1800 should have factory defaults for settings. (This is the standard setup procedure for all Ubiquiti equipment)
You are kind of comparing a apple to a orange AX client devices compared to a bridged connection that is extending you network and because of that duplexing the speed is cut in half if I’m not mistaken.
Also operating in Wi-Fi bridging mode, wireless APs generate substantial network traffic. Wireless clients connected to these APs usually share the same bandwidth as the bridge devices. As a result, client network performance tends to be lower when the AP is in bridging mode than when it’s not.
Number of people complained about it
It could be so many different things I would start with defaults then start making changes.
dmesg | grep -i Qual
returns Qualcomm Technologies, inc IPQ6018/AP-CP03-C1