If you specify a CID font directory as an argument, mkcfm will try to go through the subdirectories of that directory, and create one summary of font metric files for each CIDFont file and each CMap file it finds. mkcfm will calculate the summaries of the font metric files stored in AFM subdirectories of the CID font directory.
Those summaries are needed by the rasterizer of CID-keyed fonts to speed up the response to X font calls. If those files do not exist, CID rasterizer will have to go through usually large font metric files, and calculate the summaries itself each time the font is called. You will notice a substantial wait on a call to a large CID-keyed font.
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