Red Color Of One Image To The Alpha Value Of The Second?

I am trying to create a function that would take 2 images. 1 of the images is a grayscale image and the other is a normal image. What i want to do is apply any of the R or G or B of the grayscale image (in grayscale image r=g=b so it makes no difference) to the Alpha of the second image. i DID succeed on it. However the code is EXTREMELY slow. It takes 1 second to perform it. And i have to perform the function atleast 30 times a second (it will be outputting to a Decklink card). Here is the code:

public function createOverlay(ByVal Main as Bitmap, ByVal Key as Bitmap) as Bitmap
For index as integer =0 To (Key.Width*Key.Height)-1
Dim crntX= index Mod(Key.Width)

[code].....

This function will have to run infinitley until the user presses stop and keep updating the output buffer. How can i make it faster? ( i need at least 30 executions per second)