The lag seems very intermittent.
Mine is terrible however even minimising and maximising finder shows lag, not impressed at all becoming ever so more tempted to install ML beta instead. I hear this fixes some of the problems.
I am however waiting for a replacement to be sent out to me for an entirely different problem (rattling,creaking inside)
I have a 2.6/8GB Ram RMBP, i haven't witnessed any 'lag'.