In preparation of my new seminar PDF Color Management I have tested the image compression before and after color conversion in the most popular PDF tools.
In highend PDFs destinated for quality printing the images are usually compressed using JPEG maximum quality. Before color conversion the images must be decompressed and afterwards compressed again. Acrobat always uses JPEG medium quality for this recompression. The consequence is that there is an (unexpected) loss of quality of the color converted images (see table).
Callas pdfToolbox and Enfocus PitStop do a much better job. There the JPEG compression after color conversion corresponds to the one before color conversion:
This loss of quality in Acrobat can be avoided if the images are converted to ZIP compression before color conversion. For this purpose there is a new preflight fixup in Acrobat DC called Downsample/compress color images.
We used to have the same problem after transparency flattening. But for this feature in Acrobat X an option to define the compression settings after flattening was introduced.
Adobe should also do the same for color conversion!