This is a cheap, fast blur (right) that gives a less refined result than the Gaussian Blur (left).
The Box Blur is an imperfect but reasonable substitute for a basic defocus. The highlight blowouts that are a feature of a real defocus can be faked by duplicating the un-blurred image and then doing an extreme Curves highlights clamp (i.e. whiten the highest points and blacken everything else). This can be set to screen over the Box Blurred result.