@David-TP 

You nailed it with the topology.

I've got a fibre-to-RJ45 converter set up at ((link1)) that I tested against my laptop with a USB-C-to Ethernet adapter rated for 2.5G - speed tests on Fast.com hit around 1.5-2G on that link as they are capped by my internet plan.

I then tested at ((link2)) by plugging the same USB-C-to Ethernet adapter into the second 2.5G port in the X50 Pro node and speed tests consistently appear to be capped at around 1G. The switch I have set up is a Mokerlink 5 Port 2.5G switch, but i seems to be performing to spec (as I was able to isolate the capped speed to the port on the X50 node itself). I also tried flipping the ports and the same speed was observable.

Previously, I'd be able to hit ((link1)) speed at ((link2)) consistently without issue.

As for any abnormal messages, none! I've got notifications set up with the app so I figure I should have gotten a ping, but I did not, and nothing's visible in app, either.

Thanks for your help!

  @David-TP 

I've got an iPhone 15 Pro and it's able to hover around 1G at times, but no more. More tests below!

As for re-setting another Deco as main, more tests below:
 

• I set another unused node I had (let's call it node 3, with the older firmware) as new main and speeds went back up to what's expected, there.
• I ran a test against node 2 (ethernet-backhauled, original non-main) and line speeds were also what I'd expect (hovering around 1.5G to 2G).
• I then ran Node 1 (original main, with latest firmware) as a non-main node and line speeds were also what I'd expect (hovering around 1.5G to 2G). This to me would indicate that there isn't a hardware issue, at least, as it is capable of hitting those speeds.
• I then re-set the original node 1 as main, and re-ran speed tests against it again and observed the same speed caps I'd originally reported.

To me, this doesn't smell like as much of a hardware failure as something hindering performance when a node is running as a main router.

One last test:
 

• I disabled QoS and it looks like speeds picked back up to what i'd expected! Kinda funny, though as I did have my testing laptop listed on there (and max bandwidth of 2500 Mbps).

So, to summarize, it looks like blowing away my old QoS config seems to have fixed the issue I was seeing, there. Still seems like a worth report to escalate if it's reproducible on y'alls end.

Thanks for your time and help!