You haven't explained how you done the test, was it wifi or ethernet? If ethernet then connect directly to the hub via cable or wifi and go to samknows.com/realspeed to which it'll say the speed to both device and the hub itself.
** I work for VirginMedia but all opinions posted here are my own.
What speed configuration is showing in the hubs admin pages?
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how are you testing the speed? have you tried a different speed test service?
as you're on the gig service, VM will want to know what the speed looks like using samknows speed test - unfortunately, this will only work with the hub4 in router mode, but it's probably worth doing to just see what it's reporting