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The encode times from the link hash (733fec0) look the same as version 3.2.0 (slower than 3.1). Here is what I see on my x64 machine:

OpenCV version : 4.0.0-pre
Avg time PNG: 0.10924sec  JPG: 0.01666sec
Avg size PNG: 1925061     JPG: 1509126

The I traced down the commit which changed encode times from the link hash (733fec0) look the same as version 3.2.0 (slower than 3.1). Here is what I see on my x64 machine: timings to https://github.com/opencv/opencv/commit/1b22783d4636a74512171d67b95e6482f73cd897

OpenCV version : 4.0.0-pre
Avg time PNG: 0.10924sec  JPG: 0.01666sec
Avg size PNG: 1925061     JPG: 1509126

I traced down the commit which changed encode timings to https://github.com/opencv/opencv/commit/1b22783d4636a74512171d67b95e6482f73cd897

The fix for me is to set the change doesn't actually cause any functional change (the default compression strategy explicitly to IMWRITE_PNG_STRATEGY_RLE, which was the 'as coded default' value should be the same just using a different #define), so it looks like before this is probably a result of Z_RLE being aliased to a different value in my build environment.change.