I just upgraded to Windows 10 Pro from Windows 7 Pro. Mathematica 10.2.0 was working great on Windows 7. But AFTER installing Windows 10 is when Mathematica developed the same symptoms: Very slow, always displaying "Formatting notebook contents". This is essentially the reverse of Heiner's situation.
I have also uninstalled and reinstalled Mathematica 10.2, but it did not resolve the problem.
Otherwise Windows 10 is rather nice and it will eventually become the next standard, so I have no desire to go back to Win 7 just so that Mathematica will work. Beside, there's no guarantee since this problem apparently exists in various Windows versions.
Does anyone else have this problem after upgrading to Windows 10?