you mean once YOU use it. I have yet to experience anything resembling lag on the device. It runs every game I've thrown at it nice and smooth. Cut the Rope, Jetpack Joyride, and so on. Local 1080p video? Likewise. Streaming 1080p video... likewise. The only thing I have noticed is that if you have a lotta apps open it will drop frames when flipping between them if you go really fast. So, there may be issues with some multitasking. But, it drops the frames, so instead of sliding in apps just pop into frame instead.
Outside of that, it's been pretty darn king. But, that's my experience and perhaps somehow I got lucky and got a stellar device. But, their evidently are performance issues for quite a few people as MS has stated a fix is coming. Again... my experience isn't the same for everybody just as yours or others aren't the same for everybody else.