Similar here, reading the file from stdin on a 2020 MBP on macOS 11.2.2 is incredibly slow.

    int main() {
        std::ios::sync_with_stdio(false);
        std::string str((std::istreambuf_iterator<char>(std::cin)),
                         std::istreambuf_iterator<char>());
    }

takes around 8 seconds to run, no counting at all.

    int main() {
        std::ifstream f("kjvbible_x10.txt");
        std::string str((std::istreambuf_iterator<char>(f)),
                         std::istreambuf_iterator<char>());
    }

runs in ~270 miliseconds. Factor 30x, yikes.