It's not a lot of time, but yes. it takes time.
In this example the text "hello" would already live somewhere as a constant value,
but str would have to be set to point to it at runtime.
and the value 3 would have to be stored in num
If you're feeling adventurous, try dissasembling your executable with objdump with and without extra variables. You'll see that there are extra instructions inserted by the compiler (either setting a register or doing a load operation) when you create more local variables in your function. Every instruction takes nonzero time...
Though is a small time isn't it depend on the task you are performing? Consider a video streaming application which require faster method invocation??