Think of Wi-Fi as mains water coming into your house and the fire brigade connect 10 hoses to the local water mains.
If they don't squirt water you can have a decent shower, if they start squirting the shower flow decreases, add a few more fire engines squirting water and your shower reduces to a dribble and the flow from each fire hose goes down as well.
Just a few things that can also affect Wi-Fi in the average home …
Mirrors, furniture, fish tanks, people, radiators, walls (some more than others), flooring / ceilings, cordless phones, baby monitors, microwave ovens, fridges & freezers, TVs, next doors Wi-Fi and Wi-Fi equipment, DAB transmitters, the list is endless.
Many people have accepted that the VM Wi-Fi is poor and have purchased third party equipment that is much, much more powerful than the VM supplied kit and once set up have wondered why they waited for so long.
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BT Smart Hub 2 with 70Mbs Download,18Mbs Upload, 9.17ms Latency & 0.35ms Jitter.