I believe the answer is : Not a whole lot, 3 would be the fastest however. The reason you'd want your code in a code behind file is it makes your life easier as you don't have to hunt through your aspx page looking for your code or recompile it every time you make a change as you would with a DLL. I would also consider it to be a little bit more secure.
PS Why don't you test it yourself? Just measure the ticks it takes to compile.