Thanks for the tip, always nice to see the author of tools commenting on hn :-)

The (old) version of parallel packaged with Ubuntu 16.04 (linux subsystem for windows) - doesn't have --pipe-part -- but running from upstream, the speed is more reasonable:

  $ time (./parallel-20170522/src/parallel -a ngrams.tsv \
    --pipe-part --block -1 -j4 mawk -f map.awk \
    | mawk -f reduce.awk )
  max_key: 2006 sum: 22569013

  real    0m2.265s
  user    0m4.672s
  sys     0m1.672s

(Tried a few variants with/without -jN -- and this seems typical for the fast end of the spectrum).

  $ time (cat ngrams.tsv \
     | mawk -f map.awk \
     | mawk -f reduce.awk )
  max_key: 2006 sum: 22569013

  real    0m3.472s
  user    0m2.891s
  sys     0m2.406s

[ed: btw, did a double-take when I saw your Gnu Privacy Guard id: 0x88888888 :-) ]

--line-buffer may or may not give additional speed up.