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| p.1 #3 · Changing color bit depth during processing |
Stay with 16 bit at least until you have finished all significant edits that affect the number of tonal levels or colour levels either overall or in a particular part of the image (exposure, contrast, WB, curves, etc.). If what you've then have fits nicely into 8-bit space then you should lose relatively little by converting to 8-bit before doing relatively subtle edits.
Is your 8bit file that takes only seconds to process a jpg, or tiff ? Compression on a converted jpg may be having an influence. Otherwise you should only have halved the amount of data to be processed and not see such a great change in processing time. [no experience with this - just supposition]
Also, is your "10mg" file 10 megapixels or 10 megabytes ? How big is it in Ps memory after raw conversion ? That will give you a better idea of what amount of data is being processed.