I'm assuming that you're using Internet Explorer
Browsing is now happening via the webcache.bt.net server. If that is slow, then browsing will be slow. It probably will be slow. It might be very very slow.
When the problem clears (it usually takes some days, you'll want to stop surfing via the proxy. To do that, go through the same procedure, but UNCHECK the proxy server box.
Here's some other addresses you can use instead of webcache.bt.net,
with the city they serve given in brackets at the right (you
don't put that city into Internet Explorer).