Not sure. But web implementations are often highly optimized given just how much they get run. Perhaps one of the filters short circuits and does no work on black pixels because, I dunno, what’s a Gaussian blur of a white circle on a white background?

It’s based on compression. The paper explains how the GPU stores pixels in memory in a losslessly compressed format in order to speed up data transfers. And the filter stack is designed so that a dark pixel input produces an all-black output (very compressible), while a light pixel input produces noise as output (very incompressible).

Link to original paper:

https://www.hertzbleed.com/gpu.zip/GPU-zip.pdf