[Q] Differences between the stock 4.1.2 and the custom KitKats

Are there any noticeable improvements (performance-wise) in the custom KitKat ROMs and the stock 4.1.2? I mean, are there any changes that you can feel and see, or is it just a new android version with fancy new features?

Also I'd wait until a stable version of OmniROM or CM gets built until I try to flash my device.