The simplest blur available, and also the blur that renders quickest.
Simply turn up the blur. When quality is set to best, it has a similarity to gaussian blur.
Amount: Adjusts the blur for the image.
Blur Dimensions: Allows for just horizontal or just vertical blur, as well as combined.
Repeat Edge Pixels: This will repeat the edges of a blur to the edges of the frame, rather than blur them add create a vignette around the effect. The example images have ben sized smaller so that the effect is visible.