Pedantic: No. If you write

  #include “studio.h”
  int x;
  printf(“%d”, x);

the compiler knows the types ahead of time, too, so that cannot explain any speed difference (and yes, C compilers do try to avoid the generic printf in the library. See http://www.ciselant.de/projects/gcc_printf/gcc_printf.html for examples)

(The #include is essential. With it, the compiler can know what printf does, and, in theory, optimize it to a int-to-string conversion and a puts system call. Without it, it cannot, because it doesn’t know what the printf function does (in this case, it is easier to generate fast code for the compiler if it doesn’t have the source available than when it would see source code for a function called printf)

Utterly pedantic: no. As given, both are undefined behavior, and it is trivial for a compiler to discover that.

> and yes, C compilers do try to avoid the generic printf in the library

That is true for certain format strings (containing only %s or %c conversions), but for %d GCC and Clang don't seem to want to call a print_int function, possibly because there is none: https://gcc.godbolt.org/z/rltFse

Yes, but only if you first call

  ios::sync_with_stdio(false);

https://en.cppreference.com/w/cpp/io/ios_base/sync_with_stdi...

If it weren't for locales, probably yes ;P