I doubt the update to KitKat will do any good to the (IMHO overrated) lag complains. The lag comes from TouchWiz, a piece of software so bloated with features that is almost every single time, slow and yes, laggy to the picky. However, the beauty of Android is that you don't have to settle for a could-be-better piece of software, so if the lag is a concern, you should use a different launcher (most prefer Nova) and problem solved.