[lightcatcher]

I have no experience in PNG encoding, but found https://github.com/brion/mtpng The author mentions "It takes about 1.25s to save a 7680×2160 desktop screenshot PNG on this machine; 0.75s on my faster laptop." which makes me think your slower performance on smaller images either comes using the max compression setting or using hardware with worse single threaded performance.

Although these don't directly solve the PNG encoding performance problem, maybe some of these ideas could help?

* if users will be using the app in an environment with plenty of bandwidth and you don't mind paying for server bandwidth, could you serve up PNGs with less compression? Max compression takes 15s and saves 35MB's. If the users have 50mbit internet, then it only takes 5.6s to transmit the extra 35MB, so you could come out 10s ahead by not compressing. (yes, I see your comment about "don't say to use lower compression", but no reason to be killed by compression CPU cost if the bandwidth is available).

* initially show the user a lossy image (could be a downsized png) that can be quickly generated. You could then upgrade to a full quality once you finish encoding the PNG, or if server bandwidth/CPU usage is an issue then you could only upgrade if the user clicks a "high-quality" button or something. If server CPU usage is an issue, the low then high quality approach could let you turn down the compression setting and save some CPU at the cost of bandwidth and user latency.