It depends on how you use it. The Shadertoy examples demonstrate that it's possible to do a whole scene as one shader, but I agree the performance is not great

It depends on how you use it. The Shadertoy examples demonstrate that it's possible to do a whole scene as one shader, but I agree the performance is not great for complex scenes. This one is intended to be used as part of a 2D renderer. It would be easy to write this up as a fragment shader (and the parameter calculation can be done in either the vertex shader on the CPU). I believe, without having tested it yet, that it would be lightning fast, especially because the heart of the erf approximation can be done with inversesqrt. I'm also working on a compute-centric renderer, and for that I expect it to be even faster. There, I'll break the scene into tiles, and for each tile there can be an analysis of what's inside. So tiles on the interior of a large blurred rounded rectangle can be solid colors, the edges can just compute a 1d function, etc, and only the corners with the full version. Keep in mind, what's done in many, if not most, rendering pipelines is to render the rounded rect into a buffer, run a Gaussian blur shader over that buffer into another one (often two or more stages for the separable version of the kernel or something like a dual Kawase blur). Even the simple fragment shader version should massively outperform that.