Addendum: I guess parallel is not a "drop-in" for xargs by default, but more like xargs -n1, as has been mentioned elsethread. You need to use -X (that is a capital 'x' as lowercase 'x' means something else following the xargs CLI syntax, but not its semantics, LOL [1]).

    $tm parallel -X0j16 grep -l foofoo < ../f  # 2.33 *SECONDS* wall

So, GNU parallel is "only" 20X slower than it should be and "only" 3X slower than serial xargs on a 16 core box (on bare metal, not in the cloud, by the way). Still pretty awful unless there is another "--dont-burn-down-forests-for-electricity" flag I'm missing.

[1] https://unix.stackexchange.com/questions/273170/gnu-parallel...