Riddle me this, If this particular user is able to get his normal speeds via the Xbox one X every time using just the modem but can't get his full speeds when using the XR500, it must be an Xbox one issue??? That makes no sense. I use my Xbox one X all the time to check speeds and is accurate every-time. It is even more reliable than my PC.
I mean we download games that are over 50 gig and if the XR500 can not use our ISP full speeds to do it that is an issue, don't you think?
The reason could be that antibufferbloat is set to always on, the sliders are maybe set to 70% (recommended default) and the OP did not adjust QoS to give higher percentages to devices that needed it. So the XR500 is doing what its meant to because if antibufferbloat is set to 'always' your bandwidth is reduced by the amount set by the sliders, hence adjusting in QoS gets you what you want, or you just don't go as low as say 70% on a low speed line. Also download does not have to be as as low as say upload for gaming anyway.
So the XR500 doing is doing its job, users may not fully understand how antibufferbloat works and because nobody knows what percentage a wireless printer should have over a wireless speaker (Netgear or Netduma come on a guide would be useful for this stuff ) people just leave them all on the same percentage for both up an down and share that amongst all devices equally in QoS.
If you have 10 devices and only 70% the bandwidth of a say a 55Mbps line thats 38.5Mbps for all devices, not much really. It would be great if DumaOS could set these parameters on the fly for best throughput when gaming and not gaming, Like Streamboost does. I think if If unsure set it to maybe 87% down and 70% up or whatever works best for your line, just experiment as each line is different don't stick with defaults. I still think a guide for common devices and rough percentages for them would be helpful though. I mean if you have say 30Mbps down 10Mbps up you can always leave it on 'detect high priority traffic' so you get your full download unless you are gaming and if bufferbloat on your line is not bad anyway you wont see much effect on streaming say Netflix even in 4K anyway.