Gaussian blur filter is slow

The filter feGaussianBlur is slow due to the native implementation of the calculation of the pixel average especially with increasing zoom factors (O(n⁴)).

This can be improved (O(n²)) with some known tricks:

• assume you often calculate the average pixel values for sub-rects - how do you optimize that?
• solution:
  • create a table of the size of the image, containing the sum of the values of all pixels above and left of it (including that pixel)
  • to find any rect's average value: bottom right corner - top-left - bottom-left + top-left from table (check for off by one)