My Deco BE85 has the same problem. I use a new Cat 6A cable to connect it to my 10G modem. Speedtests on several sites never exceeded 2.5Gbps, so I guess that's the WAN port speed it negotiated at.

I really would like this to be fixed!!!

And another suffering the exact same issue with the same error message since the latest firmware update.

We don't have gigabit ethernet but we do have 150mbps coming into the house that the deco m5 units are throttling down to a max of 15.

TP link you need to accept there is an issue get this sorted.

  @Keepflying 
Any update on this? I'm having the same problem with 4 deco M9's.
Have even switched some deco's around, but speed cap remains. Have to sell them and buy a different brand if this stays.

@David-TP I'm experiencing the same behavior on my M9 Plus V1 devices. After a reboot the 'full' bandwidth / speed from my ISP (500Mbps) is available throughout the network. Then, after a while it gets "throttled" at about 95Mbps. I verified (multiple times) the bandwidth on my ISP modem is still 500Mbps when this happens. All cabling is verified (and fine) as well. I tried switching to a different Deco unit as main device; without seeing improvements.

As I am a developer myself; I do think this is a software-bug within the Deco firmware. It is present in both v1.6.2 and v1.6.3 (the latest version at the time of writing).

Can we do anything to help TP-Link developers figure out the problem... and work towards a solution?

Thanks in advance. 

  @Braindeer @SHN Try this. Go to the Deco app - More - Advanced - NAT forwarding - SIP ALG and disable it. Run the speed test again. I turned mine back on afterwards and so far it's still the same but if it drops again I will leave it disabled. Not sure what it is though or if a firmware brought the option in?

  @Cappuccino 

Tplink agreed it was a fault with the device and offered to replace the units.

Unfortunately mine were out of warranty period so they refused to replace them.

I binned them & bought ubiquiti instead.