I am also having an issue with this, it takes 3 seconds(sometimes longer or less) for my mj7u keys to work on my MacBook Pro (Retina, 15-inch, Early 2013). I have cleaned the keyboard ribbon and it is not that. I have tried mouse keys, PRAM reset, NVRAM reset and clean install. I have replaced the battery recently and this began shortly after, There was spillage in the laptop when I got it, but this has not effected the laptop before and like I said the keys work intermittently. Any help would be greatly appreciated?