SVG is too slow for very large data sets as you pointed out in this helpful example: http://www.highcharts.com/demo/heatmap-canvas

I realize that highcharts has moved to using SVG, but it would provide the user more flexibility if they could choose the renderer (SVG or canvas). Extending highcharts to use canvas.

In my usage case, I have anywhere from 3,000 to 30,000 data points in my map. Switching to canvas made the map render over twice as fast (and is much more responsive)...but it has been a lot of effort to get the desired features working.

(BTW, I believe Zingcharts has this feature.)