Comment by xashor

In J (which might be slower than K) with excessive comments for a one-liner:

    echo@> (2{ARGV) -.&([: <;._1 LF, 1!:1) (3{ARGV)
    NB.    2nd arg                          3rd arg
    NB.              g&f execute f for each, then g on both
    NB.                              1!:1 read file
    NB.                          LF, prepend newline
    NB.                 [: <;._1 split based on first char
    NB.             -.  remove right elements from left array
    NB. echo@> echo each line
    exit 0

On two ~1.6MB files with ~15k lines (both the same except 3) I had lying around:

    $ time j9 -c ./pseudo_grep.ijs test_b test_a
    …
    real   0m0.064s
    user   0m0.032s
    sys    0m0.017s
    $ time grep -vf test_b test_a
    …
    real   0m5.815s
    user   0m5.234s
    sys    0m0.576s

Note that most of the script is for loading each file into an array of lines. Most work is done by -. on the two arrays, which is exactly what you asked for, e.g. 0 1 2 3 4 -. 2 4 is 0 1 3. https://code.jsoftware.com/wiki/Vocabulary/minusdot#dyadic