I am not being a hater. I work for AT&T and have used all the Samsung Android phones and others as well. We sell a lot of GS4s, and all of them I activate have some sort of lag. So I know my phone isn't defective.
I've done a factory reset, and I use Apex launcher. I also disabled all the bloat, and installed the AT&T update. I did that right out of the box. And considering all the forums have a ton of posts about lag issues, and the reviewers say the same thing, I doubt I have a bad one.
The basic lag with windows opening and closing is livable and doesn't bother me much. It's just the camera lag and especially the keyboard lag that is killing it for me. I text a lot and the the keyboard skipping hits even though I know the phone registered that I touched the screen. If I type slowly isn't as bad, so I can tell the software isn't keeping up with me.
For people that say that there is no lag either type very slowly or have very low standards. My LG Optimus G Pro runs circles around the GS4, and that's an LG! Samsung should be ashamed of themselves.
I am holding out hope that Samsung optimizes the code and releases an update. But after the GE GS4 gets released I am planning on putting it's stock ROM on my GS4, which should run really good considering it will be stock.