ZH is really fast both in terms of the speed in which heatmap is generated
and of the performance when displayed by a modern browser. This is because it
draws the map using a bitmap format with each pixel representing a square in
the heatmap. This bitmap is then encoded as base64 and embedded in an <image>
tag in an SVG.
You browser won't break a sweat with a 10-million square heatmap.
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