I personally haven't noticed any difference in performance, nope. I don't know what KitKat was meant to improve. If anything, the battery is worse when used frequently although standby time is exactly the same.
Galaxy S4 Octa here, so far I noticed a snappier UI and more RAM available at any given time compared to 4.2.2 and 4.3.
Screen also got a touch of improvement, it looks sharper and more vivid, other than that not much else.
Haven't noticed a damn thing with my AT&T S4 except for standby battery time improving over 4.3 for me. I swear to you all I could unplug my phone from the charger and let it sit and it would be dead in 4 hrs...that's with no apps running or even mobile data turned on. Basically everything was turned off. Now when I go into sleep mode it kind of freezes the battery where it's at. Still drains like a storm pipe whenever I'm using it though
i did notice significant improvements on my Moto G!
i'm on the runtime ART and the phone seems a bit smoother and faster than before! also i found Chrome to be a lot better on 4.4.2...when i had 4.3 it was rly laggy but now it's rly smooth!
camera improved a lot too and now it's pretty decent and i think it now follows the phone standards!
battery life has improved for me, i easily get 1day+ on moderate use!
aaand that's the most significant improvements i noticed!
maybe those with a 2Gb RAM won't rly feel any performance difference because Kitkat optimizations were more aimed to devices with low RAM! so i guess pplz with 512Mb - 1Gb RAM will feel it the most! (if any 512Mb phone gets Kitkat lol which was Google intention, that even older phones can get the most recent updates)