Or maybe don't even Zopfli it until it gets 100 some hits. Cost some amount of cents of CPU time to do the compress... unless maybe you made the uploaded so it.
Yeah the smart thing to do is schedule "important" user submitted images for heavier recompression, but even then you might get in trouble for very large PNGs, and need to gate based on dimensions. For huge PNGs I am not sure the 80x - 160x time penalty is tenable, you might be talking 30 minutes in some cases.