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.
I went to high school in a science magnet program in Texas. Students from 3 high schools were eligible for the magnet program, and the magnet program was housed in one of the 3 high schools. Our math+science classes were in the magnet program, but our English/history/PE/art/all other classes were in the host school. The program made up about 10% of the host high scool, and students in the program were counted as part of the host school for purposes of university admission.
This understandably made people unhappy at the host school - ~7% of the class is academic high achievers from out of the school zone who take most of the admission spots reserved for the top N%.
I don't know of any cases of parents moving to avoid the extra competition, but I probably wouldn't have heard of that if it happened. I do know of some people set on going to UT who did not apply to the magnet program so they could have less competition.
The point here is you don't need to move to a rural area to decrease school competition. There are plenty of cases where you can move a mile to get into the zone of a less competitive school.
Wow. These videos bring home how insane the rescue was. I could not imagine anyone going through those tunnels with zero visibility and diving equipment, let alone while moving a sedated person along as well.
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.