but my prog. takes lot of time in win-api function call

If your program takes a lot of time calling the function, as I suspect, then you're barking up the wrong tree.

The difference between the two is the declaration of the API function, which happens one time in your script (if you're doing it right). The difference between should be no more than, say, 100µSec. At worst.

The calls to the function should be (close to) identical under the hood, so they should take virtually the same time.