Could you satisfy our curiosity regarding the hardware being used? MD5 speed ought to be in the ballpark of .5 GB/sec for modern cores (single thread). What bandwidth does your disk subsystem sport?
Regarding the problem:
$ find . -name '*.md5' -print0 | xargs -0 -n1 -P4 md5sum -cShould run four threads of md5sum checks in parallel (provided separate checksum files exist for each image file).