It would be nice if Samsung didn’t so heavily modify the stock Android UI with TouchWiz, but they do. What’s especially frustrating is that even with impressive hardware specs, TouchWiz slows down the S5. Adding a cherry on top of that is the fact that you can’t really get rid of it. You can go into settings and disable various functions, which will help. In particular, you can disable “My Magazine,” which is, in my opinion, the worst optimized part of TouchWiz. Just tap on an open spot on your home screen, then tap “Home Screen Settings,” then uncheck “My Magazine.”
You can also try a different launcher to change the user experience. Nova launcher is one I like, other people prefer Go or Apex. You should be able to find them all on Google Play. A new launcher will make the phone feel and look quite a bit different, and may be enough of a change to make you happier with the S5.