The final filtering pass are technically layers, overlayed on your final image. That being said, if you're using more than just GISD, I'm pretty sure there are 16,000x16,000 rasterization being generated ontop of the final render tif, and then applied to your image. This is where people that don't have enough RAM often crash when their render is already filling RAM. Additionally, some of the filters like Bloom/Atmo bloom are doing calculations to a degree over a basic image effect like Starburst.