Hmm, I'm on BT myself and not noticed any lagging issues.
My brother did and he mostly played FPS games, but the "lag" he was complaining about was more due to his monitor's refresh rate (60 was too low for him).
Could maybe do a clean Windows 10 install and see how it goes from there?
Yeah I guess that is the only option, now as far as I know you cant do a fresh install if it's a upgrade so anyone know the best place to get Windows 10 or is it best to just buy a retail version?