I've tested it against a 800 page compilation and didn't face any issues : https://github.com/captn3m0/pdp-book. Takes ~2 seconds for a fresh build on my system right now.

    time pystitcher --verbose complete.md  output.pdf
    [2021-12-31 00:00:48] INFO:pystitcher.stitcher:Deleting temporary files
    real 2.244 user 2.160 sys 0.063 pcpu 99.07

I've run it against entr[0] for live-recompilations, and it was pretty good.

[0]: https://github.com/clibs/entr