Simply because Python is a more high level language and has to do more different things on every iteration (like acquiring locks, resolving variables etc.)
“How to optimise” is a very vague question. There is no “general” way to optimise any Python program (everythng possible was already done by the developers of Python). Your particular example can be optimsed this way:
a = 1
That's what any C compiler will do, by the way.
If your program works with numeric data, then using numpy and its vectorised routines often gives you a great performance boost, as it does everything in pure C (using C loops, not Python ones) and doesn't have to take interpreter lock and all this stuff.
