Yeah because the GPU has special hardware which you can take advantage of for an optimized box filter.

https://www.rastergrid.com/blog/2010/09/efficient-gaussian-b...

That link is not a box filter, as it still uses weights to approximate a gaussian convolution kernel. It just uses some special hardware to do less texture fetches. But that is a constant 2x improvement over the full 1D convolution, not the box filter O(1) approach that the article suggests that browsers are using.