I asked github copilot to add a latexmk -parallel[=n] option and ate dinner and came back and tested it and it seems to work fine. First it just allowed various top-level .tex files to be processed in parallel. Then I asked for other stuff to be concurrent too, and also to stop mixing up outputs and buffer them instead, and it did that. Then I told it I was worried about errors from interaction of multiple subjobs generating the same intermediate file, so it added some flock'ing. And I tested it some more and it seems to work fine.
https://github.com/barak/latexmk branch copilot/add-parallel-option-to-latexmk "If you can dream it...!" --Barak.
