Techspot compares the performance of Windows 7 and Windows 8, you can view their conclusions over here. The reporter notes Windows 8 is on par and occasionally faster than Windows 7, but overall there's little difference in performance between both operating systems.
Back when Vista first arrived I remember comparing how it performed to XP and being extremely disappointed with the results. Vista was generally rough around the edges and that included drivers, so gaming and productivity applications were more often than not slower in the new OS. Microsoft's PR machine has been hard at work over the past few months, trying to explain the numerous improvements Windows 8 has received on the backend. The good news is that it shows.
For comparing Windows 7 and Windows 8 we will measure and test the performance of various aspects of the operating system including: boot up and shutdown times, file copying, encoding, browsing, gaming and some synthetic benchmarks.