@Whitfield Hi there. I was just browsing the forum and found your thread. I don't have much to add but some stuff I would consider. I did briefly gloss over so it might have been mentioned by others.

"which is just under my desk". You should never put a router or anything like that under a desk. It's like you're instantly putting another wall in front of the router that it has to penetrate. You also don't mention what type of desk so if it's metal you might be getting a weird faraday cage effect.

Do you have anything else to test on besides a macbook? There might be a weird driver conflict on their systems. Also having different ethernet cables would rule out bad cabling.

"Noise: -89 dBm". I don't know much about wireless signal strengths but this does seem really bad to a point you shouldn't be getting a connection. I have a feeling you're dropping a massive amount of packets. Have you ran tracert to an easy address like 8.8.8.8 or pinged your gateway ip on the macbook to see what values you're getting?

  @Whitfield Interestongly when I got back to the one x25 unit I reset and installed on the x68 network yesterday I looked again and did in fact find a firmware update. Weird, it did not find updates yesterday on either network setup when I worked on it and I specifically checked for updates.

Regardless, I updated and the unit performs the same, maybe just slightly worse dependong.  10-20 mb less speed now but that could just be time of day differences on the test servers I suppose. Firmware 1.1.4 build 20221115 rel 78784.   If that firmware has been out since November I'm baffled as to why it did not show up until tonight. No matter, it's no help unfortunately for my devices after applying. 
 

I'm testing hardwired to the x25 which is directly wirelessly connected to my main x68 Deco. Hardwired to an x68 deco unit in the same spot I get the full 500 down 20 up that this specific test network is rated for. So x68 is fine, 5400pro is even better and handles 1gb down and 40 up just fine. x20 series seems bugged currently.

A partial update here:

Tonight I decided to put my Deco Main unit in a different location for testing. Most recently I was connecting directly to the ONT with a short ethernet cable, presumably the best possible spot. But since that spot was infeasible at this moment I decided to detach my TV from its ethernet and use that as the source — at the end of a 100' cable plugging into the switch. 

I haven't changed anything on my network or on the Deco, but something sure has changed — I am now frequently getting ~380-400 Mbps over wifi with my MBP. I was prepared to tell @Thrill that the desk wasn't the issue (since it's a standing desk with open air beneath it), but now I'm wondering if that is a factor. Maybe the fact that the desk is made of 2" hardwood is part of the problem? Edit: just put the Deco back where it was, under my desk, but with my computer to the side so it has "line of sight" — back to 200 Mbps. Still, my Netgear always got ~300 Mbps over wifi, and that still doesn't negate the ethernet restriction to ~200 Mbps, which suggests that wifi connectivity isn't the primary bottleneck.

@Riley_S, oddly, my Deco cannot see a 1.1.4 update, it says it's up to date with 1.1.3, I tried twice. Maybe my v.2.6 model isn't compatible? I downloaded 1.1.4 but I'm not sure it's wise to try implementing it, the docs show warnings about that. 

More troubleshooting to come later this evening, as well as trying AP mode.

Ugh, no joy with AP mode. I now have this setup:

ONT > switch > router > Deco AP

...and I'm still getting 200 Mbps on wifi with my MBP right beside it. :/ I guess now I have to just wait to see if a TP engineer can solve this problem. 

  @Whitfield I'm sorry to hear that you didn't get any improvement, I know I saw a big improvement using AP mode, but I was primarily measuring wired performance vs wireless. For me my wired performance down tripled and my upload was 10x faster in AP mode vs router mode. 
 

Wireless was also improved, but I measured wired so no blame could be on the wireless connection itself causing the issues. Regardless, fully updated and on different networks altogether the x20 units I'm messing with are certainly still bugged for sure.
 

Hey, @SpiderLAN, I didn't actually have a spare ethernet cable in my last setup, so I couldn't test the 'wired' speed in AP configuration, but I just did and I do get much higher speeds! On wifi I'm still getting ~200 Mbps, but I'm hitting 700-1000 Mbps with ethernet plugged into the back of the Deco. Not sure how helpful a data point that is, but I am considering moving forward with the Deco despite the wifi bottleneck, since my wife and I are usually plugged into ethernet for work anyway.

  @Whitfield Awesome news, glad to hear it! If you wanted to confirm the bug you could move deco back to router mode and test wired again. For me when I do that my wired speeds go right back down to around 200mb. Which makes no sense, I was testing with one single client wired so it should not be stressing the router portion at all, which is why it has to be a bug.

Then again, you're working well now and are probably sick of messing with it, might just leave it be for a while. :)