Wi-Fi speeds vary because the outside influence that affect the Wi-Fi signal change from one minute to the next.
ISP provided Hubs are renowned for their poor Wi-Fi, and at the best of times Wi-Fi is a fickle beast that can be affected by numerous things such as distance from the Hub, building construction, other electronic devices, neighbouring Wi-Fi, basically all sorts.
VM supply to the Hub, after the Hub it is up to you.
Many on here, myself included advocate the use of a third party Gigabit capable Wi-Fi router or Mesh system.
It also depends on what exactly is being measured and how. If you're not experiencing problems then ignore what some app tells you. If you are having problems, describe them and somebody round here will try and assist.
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Thanks Mike. I do actually use a router a Nighthawk 7000. The puzzle for me is that nothing changes between my router and my device. Just normal obstacles like walls etc. This would affect general signal strength but why variability. The trouble started with starting a subscription to Britbox which is not available on Roku. I have had to use a Chromecast device. Periodically the signal dropped and the programme was lost, something I haven't had with Roku transmissions. I'm wondering whether the problem could lie in the device I'm using to cast to the Chromecast , a Samsung Tab S5e ie not casting very well