options processing is pretty slow right now, mostly because the
markdown-it-py parser is pure python (and with performance
pessimizations at that). options parsing *is* embarassingly parallel
though, so we can just fork out all the work to worker processes and
collect the results.
multiprocessing probably has a greater benefit on linux than on darwin
since the worker spawning method darwin uses is less efficient than
fork() on linux. this hasn't been tested on darwin, only on linux, but
if anything darwin will be faster with its preferred method.
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