@akolobych 

Hello, please help verify some details, and we will try to fix it together:
1. Without any other devices connected when the speed on the computer drops significantly, please connect one computer to the AX1500 directly and go to speedtest to run the test for 3 times and show us the screenshots of the test results.
And please tell me the wired link speed at that time, how to get that? You can refer to the below link.
https://www.tp-link.com/faq-2265.html
2. What is the speed tested on the Tether app? It might be different from that on your computer, we may think the Tether speed is more like the connection speed between the router and your internet, which would be different from the speed you can get on the computer, while we can verify it first.
3. Do you enable any QoS setting on the router? Kindly show me the QoS setting page on your AX1500, try to disable it then check again.

4. Feel the router when the speed drops significantly, if the router is hot or lots of heat coming out, put the router in a well-ventilated location or mount it vertically on a stand that permits the free flow of air, or use a fan to cool the router, a regular fan or USB fan is fine.
5. Set the Wi-Fi Transmit Power to middle or low (if the setting is currently set to high), and see if this helps.