Thanks for the suggestions, @Riley_S, but I'm afraid that didn't get me anywhere. I set the QoS to 1024 up and down (the max allowed, and my network occasionally does hit that mark) and chose my MBP as one of the "fast" devices, but the speed test didn't change, still about 200 Mbps. 

I'm a bit confused by your second suggestion. As a web designer, I'm familiar with using Cloudflare as a CDN in DNS settings, but I've never heard of applying anything like that to a local network. I'm not sure where in the app I would change that.

Thanks for the additional ideas, @Riley_S! That's really interesting re: the DNS, now I'm remembering the whole '1.1.1.1' thing that appeared a few years ago, I think I set that on whatever router I was using then and totally forgot about it. :) I've changed it as such (with 1.0.0.1 as the alternate) but it doesn't seem to have had any effect in terms of speed. 

I had taken for granted that replacing the old router with the Deco with the same wire in the same place should produce the same or better results, so I failed to mention that I do have a switch in our network, for reasons that aren't worth going into. The setup is:

ONT > ethernet switch > router

I decided to try plugging the Deco directly into the ONT output to see if that changed anything, but this doesn't appear to have made a difference either, even after fully killing power to the ONT and restarting it. :/ I'm wondering if I should reach out to the ISP?

Edit: I've also wondered if resetting one of the other two bases and trying that as the Main unit might be a good test? That would imply that the one I chose has an actual fault in it. 🤷🏻‍♂️

  @Riley_S I just took out a x20 system for the same issues. Units say x25 on the bottom, 1gb down and 40 up connection when ran through the deco as router I get 240 wired max, same on Wi-Fi. Pitiful 4mb up max. This is with nothing at all connected to them except one desktop connected to the primary Deco via Ethernet. No QoS ever turned on.
 

When set to access point mode and ran through an older netgear router I get 600-680 down. 
 

This is new behavior, worked fine for several months, though max speed always topped out around 700 down. Recently performance plummeted, not sure if there's a bad firmware out there or what. Does not matter what unit is the primary Deco, all perform the exact same, 240 down and 2-4 up. 
 

I replaced them with a Deco 5400pro setup yesterday and the new setup works fine. I added the new units one at a time and then removed the old x20 ones using the app and everything is fine now, 970 down and 39 up in testing. As these were added to existing deco setup that means all settings are the same exactly, so it's not a setting issue, it's a firmware or hardware issue on the x20.

This was with Spectrum Cable, not Verizon. Was on latest firmware.

Edit to add more info - hard rest the x25 Deco units and took one elsewhere and connected it to an x68 Deco network. Hardwired performance on this new network also maxes out at 250mb down, though it is just a satellite unit it has a perfect strong connection and still only 250 down when on this new network. Very strange.

Oh wow, glad I didn't have a chance to do any more troubleshooting! This post is validating, @SpiderLAN, it suggests that there really isn't anything I can do at the moment, the TPL engineers need to be aware that this is happening.

That said, since I do have a router that is working (poorly in isolation, but with strong ethernet speed), I wonder if switching the entire Deco network to AP mode would get me where I want to go. Would this setup work?

ONT > Netgear router > (ethernet) Deco Main > (wifi) Deco children

I haven't tried AP mode in years and this is my first mesh product, so I have no idea if the children/nodes will work like this if the Main is set to AP mode. 

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Oh, and one edit that relates to the previous post: we don't use Verizon either, we're on Ziply fiber, so that latest firmware isn't likely to help unless it contains more than that one fix.

  @Whitfield Yes, try access point mode and see what you get. That fixed me temporarily to adequate speeds at least. In the app you just switch them to access point mode, then connect one to your router and you are all set. Takes a couple seconds in the app to do, you should see improvement I expect.

Definitely an issue with the x20 series that even hard resets do not fix. Hope they are able to update and resolve it soon, until then use AP mode to get by.

Shut off wireless altogether on your old router also, will keep it from interfering with the Decos.

Terrific, thanks guys! I'll try the firmware update first in case that does fix the problem then give AP mode a shot.