$ echo {1..5000000} > file # adjust based on CPU speed
    $ time sed -e 's/$/stuff/' file >/dev/null # logical way to append to lines
    sed -e 's/$/stuff/' file > /dev/null  2.91s user 0.09s system 99% cpu 3.007 total
    $ time sed -e 's/.*/&stuff/' file >/dev/null
    sed -e 's/.*/&stuff/' file > /dev/null  1.62s user 0.34s system 99% cpu 1.972 total

musl via busybox sed was tested to be 2x faster in the first case than in the second.

intuitively, this does not make sense. .* should be slower because it needs to match the entire string whereas $ can skip to the end of the line (since sed must already find the new line in order to run the commands).

however, glibc spends an inordinate amount of time inside of check_halt_state_context, re_state_reconstruct, and re_string_context_at, according to callgrind.

I am unsure whether this qualifies as a glibc bug or how to fix it, but I think it is useful to have on the record.