To anyone with the weird speed issue, I can confirm that deleting the tether app, resetting my AX3000 router and setting up the router again through the web interface works. Unbelievable that TP-Link isn't having this as a front and center issue listed on their product page when so many people have the problem!

It's not because of the Tether app. I'm using it even after having the problems. The problem is the QoS. What might happen is that when you use the Teather app you accidentally select an option from the QoS, that's all.

@Sevi  I am having the same problem. AX50 speed is half that of a direct modem connection. The support desk as me jumping through hoops of connecting using CAT6 cables and recording the speed screenshots. None of the above methods work for me. Seems to be a hardware issue imo. 

I have seriously spent half a day trying to get a new AX50 to perform as it should with absolutely no success. My old Netgear router was getting a little flaky in terms of dropping clients, so I tried to swap it out, but this AX50 is incredibly slow and none of the QoS, Tether, or other suggestions here seem to be a factor since I have done multiple factory resets and tried literally every combination of settings to improve results. Even wired speeds are a fraction of what I had on the Netgear.

Netgear speedtest results: 482 down / 22 up wired, 206 down / 22 up wireless

AX50 speedtest results: 92 down / 22 up wired, 91 down 22 up wireless

Some factory resets would temporarily get the speeds up by maybe 50%, but they would drop within minutes. What's worse is that real world usage seems to be exponentially slower than those speed tests indicate. Connected to the AX50 wifi, trying to download and install the tether app takes anywhere from 5-12 MINUTES. It's a ~50mb file!! Installing over the Netgear wifi finishes in under 10 seconds. The AX50 also struggles to load social media feeds. Facebook groups on my phone is like a 2-3 minute wait to refresh and sometimes won't load at all. Switch to LTE and it comes up instantly.

At this point, I don't see how it could be anything other than a hardware issue and I'm shocked that this router gets as much praise in online reviews as I've seen. I may try to exchange it with another unit, simply because there's no way this can be the norm, but it's definitely concerning to see so many other people having performance issues.
 

UPDATE: This thing is garbage. Received a new router yesterday from Amazon and it is having the same problems. Facebook and other social media apps absolutely will not load properly over Wifi. It's as if QoS or parental controls are enabled, but they are not. It is blocking that traffic right out of the box with factory settings. If I turn off wifi and switch to LTE, everything loads instantly. Going to have to switch to a different brand, I guess.

I ended up returning my AX50 and going all-in on the ASUS Zenwifi AX mesh system. So far it works beautifully; much faster speeds, coverage, and customization. 

OK, in another thread, someone suggested enabling IPv6 on the router in order to fix loading issues in social media apps. Amazingly, this seems to have fixed most, if not all, of the issues I was having despite the fact that I never enabled IPv6 on the Netgear router I am replacing.

HOWEVER, this did cause a new problem in that Netflix was not working on any LAN ports. By default, the AX50 has IPv6 set to SLAAC for LAN. Switching this to DHCPv6 seems to have fixed the Netflix issue. Cautiously optimistic that this will be the end of all this nonsense and that everything is working now. I still don't trust this thing 100%, but I really don't feel like returning it again and starting over with something else.

@PETzz agreed. I had one device in my QoS prioritized and it knocked all my other devices to half the speed of my internet which was 250 Mbps out of 500 Mbps. Thanks for the tip PETzz.

So is the AX50 not a good buy as in prospective buyers should stay away?

@Linsook I recommend you the little brother AX20...