[Asm2D]

I don't know what rasterizers you refer to with:

  "CPU rasterization is always O(N) complex (where N is number of pixels on screen)"

But this is definitely not the Blend2D case. I think you will not find rasterizers in production with such properties in software-based 2D rendering as that would be really inefficient. Path boundary matters and that is often the worst case scenario, but Blend2D does much better than this, for example.

I see no problem with multithreading, because it doesn't mean that all CPU cores will be busy with rendering, it means that the total time required to render a frame will be much lower while utilizing CPU power in a more distributed way instead of stressing a single core. You can use 2-4 threads on 8 core machine, for example, leaving the rest for other real-time tasks if required. Applications that use GPU for 2D rendering also use the full power of the GPU, if available.

Single core performance is stagnating while the number of CPU cores is increasing, so it's simply practical to design software to take advantage of that.