@Sentinel2A
You need to provide more info for someone to help with troubleshooting.
Multiple things come to mind: Deco mesh bandwidth settings restricts traffic, device negotiated 10Mbps Ethernet link with Deco, Deco negotiated 10Mpbs with router, Satellite Deco is too far from Main Deco and can't get good backhaul link to it.
1. What is your Internet speed?
2. Can Xbox report Ethernet link speed?
3. Do you have any other device with Ethernet card, such as desktop or laptop, you can use for speed testing?
4. Please share Show List screen from Deco app, just make screenshot and attach to reply. Example:
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