Fastest Available Algorithm to Blur an Image (Low Pass Filter)
The fastest blur would be Box Blur.
You can implement it using Running Sum.
I think Intel FilterBoxBorder works in that manner.

If you'd like you can do a few passes of it to approximate the Gaussian Blur.

You can also use IIR Filter Coefficients to blur the image quite easily.

You may have a look at my project Fast Gaussian Blur.