I disabled the shading rasterization with immediate returning from patch_fill in gxshade6.c . The file Bug689189.pdf renders to -r140 display in 23 seconds. Adobe does in 5 seconds including shadings.
This seems like it should be Michael's (internal shading speed). Reassigning.
Assigning to Robin since he is working in this area.
Testing this on my PC with a release version of Ghostscript; MuPDF saves the file to PNG in 3.1 seconds, Acrobat takes 5.1 to save an RGB PNG file and Ghostscript takes 7.1 seconds. Admittedly we are still slower than the others, but not *that* much slower. Especially given that the Acrobat time was taken when the Acrobat was already loaded, and the file displayed, whereas Ghostscript was timed from the command line. I'm closing this because I expect we're as good as we're likely to get.