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libcxml
openjpeg is an open-source library for the encoding and decoding of JPEG2000 images.
This page offers some patches which improve its performance slightly.
I have one solitary test: it is a 1998x1080 pixel image which I am encoding. With the patches described here, on a i3 2.27GHz laptop, an encode of the test image takes 76% of the time that it takes with openjpeg 1.5. The result is bit-for-bit identical.
The patch against openjpeg 1.5 is here.
I have a repository on github that has the patches split up into steps.