Usually each new version of iOS reduces the performance of old iDevices, Apple doing this to force users to buy the newest terminals that have the best performance. To see how much performance changes when you install iOS 6 on your iDevice, those from MacWorld they thought of running some benchmarks, and in this article you have the results. Looking at the images, you will see that in the case of the iPad 3 there is a significant decrease in performance in certain situations, but in general the rest of the terminals have approximately equal performance.
