Yes, I have. And, no, it's not relevant to the discussion of why the brush is slow.
Nothing 2+2=5 has said is wrong. It's just besides the point.
The point, which I sought to clarify, was why the Photoshop brush is slow. The comparison between the simple brush and the complex brush was never the point. Ever. I wanted the readers to fix their attention to this snippet from the article I linked:
It's a short and concise point which clarifies why something seemingly simple is slow. To me, this does answer the question "Why is the Photoshop brush slow?". The answer, in case you missed it, is "because the algorithm used to fill the brush is order n-squared complexity, where n is brush radius" (I've extrapolated this conclusion from the article, so it's not necessarily true, but to me it doesn't seem unthinkable). You can disregard the rest of the text in the article as it doesn't lead to another conclusion.
As for what I'm suggesting; None of the above. We already know Photoshop has GPU accelerated rendering, so what's the point of suggesting otherwise? However, we also know that O(n²) scales poorly regardless of what silicon it is being executed on. As for suggesting that the chosen algorithm is poorly chosen, valid, and/or complex; If I were suggesting any of those alternatives, would that somehow make it not an O(n²) algorithm? Would it modify the original point I was trying to clarify? The way I see it, the answer would remain the same regardless, which is why I refrain from such value judgements.
I don't know maybe i'm not good at explaining things(well English is not my mother language so it can be), what you quoted clearly says:
"
In its simplest form, the brush tool requires a circle filling algorithm"
That's what i was pointing since the beginning, it's easy for a simple flat round brush that draw on a single layer in normal blending mode to be fast, how can you think that the same exact operation in a program that let you personalize the brush and using many layer has a comparable complexity?
The filling algorithm of papaya probably just fills the circle covering what it's under it(=normal blending mode), on complex programs the shape can be really complex(it can even be a texture, and we are here talking about "circle filling algorithm"...), randomly rotated and many settings influenced by pen pressure and speed(some physics algorithms are involved), some brushes also have engines to simulate certain kinds of real world brushes(other algorithms) and each single pixel has to interact with many layers in many different modes, think of a textured brush simulating a real world brush with some dynamic size/rotation/alpha/etc pen settings over many layers some with normal blending mode, some with multiply, other with something else, can you imagine how many operations photoshop has to do for each
single pixel of a stroke? The canvas can also be rotated in real time so there's even some 3d involved.
How can you think that a simple brush filling algorithm can be the same as one of a complex program? That is exactly why the photoshop brush is slower, how can you even think to apply the simple papaya filling algorithm to photoshop? Flexibility and complexity always come at a cost, how can you say that all of this is besides the point?
I remembered one thing, i read somewhere that a good way to be faster with photoshop is to limit the number of layers merging the ones that are already finished, so those who experience lag try to work with less layers.