User blog comment:RoznosicielCiast/Fifth balance attempt - Champions and items/@comment-8506165-20150106223632

"  Cell Division Nice idea, but I've been musing over Zac for a while and I'm pretty sure that's definitely not a good buff for him - the problem is that his passive is unreliable, not that it isn't up often enough. This change would just make him more overwhelming if his team is ahead and isn't much use if he's behind. If you want to buff Cell Division, you need to make it more reliable in some way - working faster, or making the bloblets harder to kill.
 * Cooldown reduced to 270  / 240  / 180 seconds from 300 at all ranks. "

More slow on Q is okay, but the main problem with Zac will always be that Riot designed him as a sustain fighter/tank, and then gutted his sustain mechanic in response to toplane Zac (contestable vs. non-contestable chunks). So now he's a fighter/tank that kills himself as a fight goes on because he rarely has the luxury of depositioning himself enough to retrieve his blobs in a teamfight situation. He now needs fights to be shorter, thus more damage, imo, or he needs some of his sustaining strength back to make up for his inability to effectively use it in as many fights.