Just a shot. I was having this trouble when I first installed Cubase 8. I finally tracked it down to the fact that mediabay had reset itself to scanning my entire C: drive and my F: drive (where I have “My Documents”) and for some reason it would never finish.
I finally reset it to the one folder I had in 7, and performance (including laggy gui) were instantly better. Though I still think 8 needs better gpu to behave as smoothly as 7. I do video editng, so I have a pretty high end card.
Maybe this will help someone.