@woozle 

You said "The 'Internet Port Negotiation Speed' is only for the internet port, aka the WAN port.  It doesn't affect the LAN Ports"

That interesting - when I first started working on this problem, I changed the 'Internet Port Negotiation Speed' setting, and the speed setting for the LAN ports changed, but the WAN port setting did not change.  Specifically, I changed the 'Internet Port Negotiation Speed' setting so 1000mBPS full duplex, but the WAN port remains fixed at 100mBps.  I can't find any other port negotiation speed settings on my AX-50.  So my experience is exactly opposite to what you said.  How do we reconcile that problem?

The other thing that concerns me is that TP-Link's response to this problem is 'try - - -'.  Try?  That tells me that they don't know what is causing this problem, and are asking users to experiment for them. 

Prior to switching to the TP-Link router, I was getting data speeds in the 15-20mBps range,  That was OK for the purposes we had at the time (basically, computer stuff).  I knew our ISP was advertising 100 mBps at the time, but that our old router was a throttle.  But then we decided to shift to streaming TV and needed to improve speeds, so I bought the TP-Link AX-50 router, and we immediately jumped to 85-90 mBps (with the ISP still delivering 100 mBps).  That was great - exactly the result I expected.

The only reason I'm looking into speeds is that our ISP has improved their service to 500-600mBps (actually measured at 580 mBps).  But because of the AX-50 router problem, the maximum speed we actually experience is 85-95 mBPS.  Fortunately, they aren't charging us a premium for that higher speed (yet), but at some point they will adjust their rate schedule and when that happens, I want to either be able to actually benefit from the improvement, or ask them to remove that premium from the service package.

But that brings me back to the 'try' - if the cause of the problem isn't known, I'm concerned that randomly 'trying' things carries the risk that a 'try' could actually degrade service.  I don't have the time or patience to have to start over rebuilding our home WiFi network to correct a problem cause when an experimental fix goes sideways. 

So tell me what the problem is, and what to do about it.  Don't ask me to participate in a trial and error program searching for an unknown cause.

My Deco's are capped at 100 Mbps when using wired ethernet. If I switch to wireless, it goes full speed. Big bug for TP Link hardware.

  @Kevin_Z 

I checked it out. Nothing from the suggestion works. It's the firmware that's causing the slowdown. Didn't happen with older firmware.

It doesn't properly negotiate a 1000 Mbps speed with the Linksys switch even if it did in the past. It limits any connection to that switch to 100 Mbps.

  @David-TP Hi, I'm in the process of moving homes right now which explains my late reply. I have moved the three Decos to the new home and will test them once I am settled down.

Your scenario is correct, main Deco connected to the gigabit Linksys switch which then connects to the respective rooms where the Deco is connected. If I connect directly to the main Deco with my desktop, I do get gigabit internet speeds. If I connect to a Deco which passes through the Linksys switch, it caps at 100 Mbps.

There's something about how the TP-Link is interfacing with the Linksys switches. The models are SE3008 and SE3005 which are 8-port and 5-port switches, respectively.

I am using Cat5e and Cat6/Cat6a cables. Nothing lower than those.

Once I am settled in the new home. I will test the speed of the Deco alone as well as with the switches. My internet plan over there is pure fiber optic at 3,000 Mbps upload and download. I know the Decos are capped at 1,000 Mbps.

  @David-TP Okay, so at my current home (not the new home where the Decos have now been relocated), I have installed my very, very OLD Linksys Velop Mesh router. Basically, I swapped out the Deco unit and put these Velop in place. My Linksys switch is still in the same connection point with the same cables.

I then did a speed test on it and I am getting close to gigabit internet speeds.

This just proves that the Linksys switch and the cables have no issues. It's definitely the Deco that cannot communicate properly with the Linksys SE3008 and SE3005 switches and cap the connection at 100 Mbps.

Modem -> Linksys Velop (previously Deco XE5300) -> Linksys SE3008 switch -> peripherals

I solved the speed issue by placing the TP-Link Deco as an Access Point. It's useless as a router to manage wireless and wired speeds.