@sca I have had the exact same problem.  I recently upgraded my service from 200mbps to 400mbps through local Cable Provider (Spectrum).  Service from the cable modem consistently exceeds 400mbps, but through the router (whether wired or wireless) I could only get 200mbps.  I went through all the troubleshooting steps in this thread, making sure that NAT Boost was on, QOS was set to default values, No devices are getting QOS priority.  I also updated the QOS speedtest values to far exceed my service (I set it to 999 down and 40 up - I get 400 down and 20 up).  None of these seemed to work.  I installed the Beta firmware and left all other settings the same, and now I am finally getting consistent results.  I get >400mbps whether I'm wired to the router or connected to either of the 5ghz antennas on WiFi.  Thanks for link to the beta firmware!  Like the others, I wish TPLink would make it available on the standard download site, but I do appreciate my router working properly again.

@Kevin_Z 

I've had the half speed issue since purchasing about a year ago. I didn't really care since 400 down / 30 up was plenty but recently I've been getting high jitter and packet loss and only 150 down / 12 up (average) with it dropping to 65 down / 0.7 up at times. 

Jitter is between 5-8ms with packet loss between 3-5% (averages). The packet loss seems to be isolated to upload. 

I've had Cox out twice now to test the lines and everything seems good. I'm on Gigablast rated for 1gb down / 35mb up.

Aside from the speed issue disscued here, I'm having jitter / packet loss issues that have become my main problem. 

QoS set to standard

Manually entered speed, using 999.0 down and 35 up. 

Only one of the 5G bands ever shows up. Speeds over Wi-Fi on 5G in same room as router are marginally better 180 down / 17 up.

After finding this thread today, I installed the beta firmware linked.

Using the beta firmware (which seem to have been in beta for a year now?) My speeds over LAN are 500 down / 25 up but it did nothing for wifi over 2.4 or 5G

IPv6 is enabled - LAN is set to SLAAC+Stateless DHCP

Changed Beacon Interval Settings from 100 to 200. Single story home, open floor plan, no walls between wirleless devices. No change.

Duplex set to 1000Mbs full duplex

Nat Boost - This is a weird one. Using the non-beta latest firmware Nat Boost usually resulted in a Strict NAT so I kept it off. Using the beta firmware with Nat Boost ON my LAN download is hitting 850mb / 25mb / 50-58ms Latency. With NAT Boost OFF there is no change to speeds between the release firmware and the beta. There is no change to wi-fi speeds with the beta firmware on either  2.4 or 5GHz.  I'm still getting 150 / 12 average.

The speed issue sucks but its the jitter and packet loss that is my major issue, I'm not sure what to adjust fix the jitter / packet loss.

Ping test from computer to interal router IP via wi-fi over 5G shows min/avg/max/stddev = 2.919/3.860/11.528/1.168 ms (Ping -c 50 destination via Termainl MacOS)

JITTER TEST 1: Late Packet Rate: 20.95% Average Latency: 252.17ms  Average Jitter: 347.79ms - Test Settings: Duration: 10 seconds, Frequency: 15 pings/second,  Average Size: 220 bytes, Acceptable Delay: 53ms

JITTER TEST 2: Late Packet Rate: 0.00% Average Latency: 33.92ms Average Jitter: 1.29ms - Test Settings: Duration: 10 seconds, Frequency: 15 pings/second, Average Size: 220 bytes, Acceptable Delay: 53ms

As you can see from these results I'm getting really inconsistent results. I've had similar results at various times of the day.

@Kevin_Z you should publish this new firmware as a new official update. after your team duble check  the changes.

@Kevin_Z Is there any hope that the final version of the beta firmware will be released, which was provided to users of this router model to solve the problem with the speed drop? We are looking forward to it and we hope the company has not forgotten about us!

@Kevin_Z Yes, the beta version you provided works correctly. There are no problems with speed at the 1 Gigabit tariff. It is very inconvenient, in the web interface on beta everything is in beta beta beta watermarks ... And because of this it is very difficult to see the interface in the device settings. So the beta firmware deserves to be released. Thanks!