I find it funny that you completely modify your phone (new ROM and kernel) and are now surprised that you are having problems. Did you have these problems when you were stock?
ODIN back to stock, root the phone if you desire, but don't mess with anything else. See if your problems go away, and I bet they will. I've had no issues with this phone and I never bothered to waste my time flashing anything to it. There is simply no need for it. Custom anything made my old S3 run like crap with much worse battery life, and I don't expect anything different from the S4. Otherwise, Wifi can use more battery, depending on the strength of the signal as compared to 3G/4G.