For those afraid of the complexity of GNU Parallel, there is the tiny moreutils `parallel` (or Tollef's parallel) that covers 90% of the common use cases with a single-page manual.
https://manpages.debian.org/testing/moreutils/parallel.1.en....
https://manpages.debian.org/testing/moreutils/parallel.1.en....
parallel sh -c "echo hi; sleep 2; echo bye" -- 1 2 3
parallel echo hi";" sleep 2";" echo bye ::: 1 2 3
parallel -j 3 ufraw -o processed -- *.NEF
parallel -j 3 ufraw -o processed ::: *.NEF
parallel -j 3 -- ls df "echo hi"
parallel -j 3 ::: ls df "echo hi"