How much faster is this:

    int run_switches(const char *buf) {
       size_t len = strlen(buf);
       int res = 0;
       for (size_t i = 0; i < len; ++i) {
         res += (buf[i] == 's') - (buf[i] == 'p');
       }
       return res;
    }

strlen() should be implemented in a pretty fast way, and after the buffer size is known, the compiler can autovectorize the inner loop, which does happen in practice: https://gcc.godbolt.org/z/qYfadPYoq