Saw the update this morning as well. The file was about 380mb, much smaller than the 4.3 update download.
Huge improvements were made on my S4 when it upgraded from 4.2.2 to 4.3, so I didn't expect the KitKat update to improve performance so much because there wasn't much to improve in performance. But, accessing the app manager and closing apps is a bit faster, ram usage has gone down quite a bit. It fixed a bug that was on my 4.3 update. Every time I would open the cam app, the transition animation would reset to 1x, I always leave it at zero. It also fixed the wifi issue, it seemed to be a bit unstable on 4.3 or I would just get faulty wifi error messages. No apps deleted on this update or anything. I'm liking the new white icons, keyboard, and the cam unlock screen. The benchmark on Antutu dropped because Samsung removed the code to run all cores. While the benchmark dropped a little bit on Geekbench, it scored much higher on KitKat than on 4.2.2.
Overall I'm very happy with this update.