Right, in my case the "few thousands" was for a single fairly simple shape (diffusion curves are basically continious line/splines for each shape border, so a single horizontal spline over the screen would generate almost 2k or 4k seeds depending on type at 1080p ).

Real diffusion curve images would multiply that by an order or two at the least depending on complexity of the image (the algo I wrote as well as JFA might have a higher fixed number of iterations but it's fixed so the number of seeds don't matter).

Besides, GPU's have turned more and more general so even if there's optimizations for the triangle rendering the compute case will probably gain even more from more recent GPUs.

Optimizing for GPU's can be funny, and I wouldn't be surprised if the break-point on where the tri-fans are faster is higher than I expect, but at the same time unless it's a tile-based renderer I'm pretty sure there will be a break-point (also tilebased renderers can sometimes fall of performance cliffs in special cases).