Do it with the GIMP → Basic Areal Effects → Blur → Gaussian Blur → IIR vs. RLE Gaussian Blurs

4.1.3.1. IIR vs. RLE Gaussian Blurs

GIMP supports two implementations of Gaussian Blur: IIR G.B. and RLE G.B.
They both produce the same results, but each one can be faster in some cases.

RLE Gaussian Blur

RLE stands for run-length encoding
RLE Gaussian Blur is best used on computer-generated images or those with large areas of constant intensity.

IIR Gaussian Blur

IIR stands for "Infinite Impulse Response".
This blur works best for large radius values and for images which are not computer generated.

Written by Shlomi Fish