I thought the secondary chip to do background tasks while in standby and for "Share" features was announced in the Feb '13 PS4 reveal. Not 100% on that.
But yes you are right, could be a number of reasons. Optimising the OS could be one and they've done it in the past so it wouldn't be a surprise. The OS was probably given a wide birth to make sure the system to run smoothly when extra features were added in the future. Now they have a good idea of what they need they can start to chop down the resources.
The OS will handle stuff in the background while playing games like apps, messaging, chat, trophies. suspended apps etc and the UI has to pop up within an instant when called upon. My PS3 UI performance took a hit when they allowed more resources to developers so hopefully they don't try and be to tight with the PS4 OS resources.
