Same problem : Boot is now X4 with Win 8.1 compared to Win8 !! (Dell XPS i7, Win Pro 64bits)
I have tried different tests (with and without TuneUp, Bitdefender, etc... looking for a new user profile, changing advanced parameters in boot via msconfig...) no major result to decrease boot time.
A lot of people are claiming (english, french...) on this incredible problem which is "indigne" (i am french) of Microsoft without any answer from their side.
I am very angry again MS !!