Wait you can PAN? I just assumed you finished the game once you filled every starting square. That's good to know lol and yes it does get laggy after a bit. I think each number and plus sign spawning takes a lot of power and since a crop spawns 4 pluses they are "super power hungry". If I am right about that (obviously I am taking a blind shot) object pooling would be a great idea. Basically instead of spawning a new text with the + sign every time you just deactivate them and then re-activate them once the next text appears and only change the number/sign on it if needed.
Really great game though. It was fun to play!