It will not slow your connection down at all.
The capacity available to you will be reduced WHEN someone is logged on to the BT WiFi connection and even that is restricted to a small percentage - the amount is dependent, I believe, on your connection speed.
I have left mine on permanently and very rarely see anyone connected to it and normally those are visitors here. I find it useful when out to be able to "borrow" a connection for emails or quick browsing and at airports when sitting there for an hours or two.
I will run a test and see how fast the BT WiFi connection actually is when taking bandwidth from my 72Mbps connection.
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