Fastest way to generate heatmap

[artemiusgreat]

The subject speaks for itself. Specifically, I have a canvas split into quadrants, each quadrant is filled with color from hottest to coolest, like this. The drawing performance stays acceptable, e.g. under 1 sec, if number of quadrants doesn't exceed 100K quadrants. When I try to render 1 mm quadrants, it degrades down to 3-5 seconds.

Skiasharp is not thread safe, but maybe there is some parallel algorithm or feature, like shaders, that I could use to produce heatmap faster? I found this sample but as far as I understand it's using some surface available only in a desktop app, while I'm trying to generate image on the server side. Maybe somebody could suggest ideas that would fit these requirements?