Ever wondered how a blur filter works?
Well, you create a matrix of weights, called a kernel, and then use those to average the pixel values surrounding your target pixel.
We tend to use a Gaussian distribution for the kernel weights, hence Gaussian blur.
The larger the kernel (radius) the heavier the blur and the more expensive the operation.
But because our kernel is symmetrical, we can actually split it up into two passes. We blur the vertical pixels and then the horizontal pixels for the same result but fewer operations.
If we are using a backdrop blur there's the added step of computing the area that needs to be blurred and then essentially rasterising that. No wonder they are so expensive.
Anyway, just thought that was interesting.