Fastest way to Convert String to Binary?

Using std::bitset would work:

#include <string>
#include <bitset>
#include <iostream>
using namespace std;
int main(){
  string myString = "Hello World";
  for (std::size_t i = 0; i < myString.size(); ++i)
  {
      cout << bitset<8>(myString.c_str()[i]) << endl;
  }
}

Output:

01001000
01100101
01101100
01101100
01101111
00100000
01010111
01101111
01110010
01101100
01100100