title: Highcharts heatmap chart labels render extremely slow
body: <p>I'm using Highcharts to generate a heatmap grid with around 10k grid cells. The heatmap renders in under a second without dataLabels. However if I enable dataLabels, which are necessary for my project, the same heatmap takes 10 seconds to render. I tried setting useHTML to true, and that renders in 30+ seconds. I'm not doing any work within the dataLabel rendering. Is there any way to speed this up?</p>
  
  <p><a href="https://i.stack.imgur.com/548Kh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/548Kh.png" alt="Image of part of grid - my grid is 270 rows"></a></p>