I didn't infer 15% from the way it was written there. But most platforms these days have some form of CRC32 "acceleration". Adler32 is easy to compute so I'm even less concerned there.

I spent a bunch of time optimising the code in fpnge (https://github.com/veluca93/fpnge), which is often notably faster than fpng (https://github.com/nigeltao/qoir/blob/5671f584dcf84ddb71e28d...), yet checksum time is basically negligible.

Having said that, the double-checksum aspect of PNG does feel unnecessary.