@gazandcat assuming you actually have bought a 500Mb/s service ie 0.5Gb/s service and you have a white Sky Max hub.
First point your Q Mini cannot connectvdirrctly to the hub as the new hubs are not compatable with the old Sky Q wifi mesh. Instead it needs to be connected to a closed WiFi signal from the main Q bix which can connect to the hub. The recommended way to network Q boxes is to press tge wps button on the mini until it starts flashing and then do the same on the main Sky Q box. If they are too far apart to connect call Sky who can provide a Sky Q booster. Note the Sky Q boxes no longer can act as WiFi hotspots for your other devices.
WiFi speeds in your home will vary due to distance and type of construction of your home. You can get 500Mb/s in the same room but a device on a different floor some distance from the hub may well only get 10Mb/s if you have an older phone. Unfortunately there is nothing Sky can do about the laws of physics. To test the speed of a full fibre connection conclusively you need a device connected by a known fast link which iis a gigabit Ethernet cable. To get the best signal you can make sure your hub is out in the open on a surface so its signal is nor compromised by radiators etc
Assuming the issue is poor wifi then like many other customers including myself you need more than one wifi source. Sky can sell you their WiFi max servicecwhich guarantees 25Mb/s in every room and can supply up to 3 pod extenders to achieve thst see https://www.sky.com/help/articles/sky-wifi-max or you can use third party wifi equipment if you prefer which is my own solution.
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65inch Sky Glass, 3 Sky Streaming Pucks, Sky Ultrafast + and Sky SR213(white Wifi Max hub) main Wifi from 3 TP-Link Deco M4 units in access point mode
