Did you get this resolved?
I reccently discovered that unless the SkyQ main box is ethernet connected to the Sky Router, and not just into an ethernet switch, it utterly floods the network with useless traffic.
It was dropping our 500MBit Sky Fibre to similar to you or worse.
As soon as we cabled the SkyQ box into the Sky Router, and not the original ethernet switch, the speeds went right up - seems the Sky router can block the excess network traffic and the unmanaged switches couldn't.
Given I can replicate this easily by plugging the SkyQ box ethernet back into the switch, I can say with 100% certainty that the SkyQ box is the issue.
