Heat maps are very slow for users with more than 1000 buildings

[Zverik]

Example area, enable heat map for Zverik or gryphon. I had to wait for ~30-50 seconds before it appeared.

[timlinux]

@sunng87 Do you have any thoughts on this? Perhaps we could implement some kind of progress indicator to make the user experience better if the waiting cannot be avoided?

@Zverik - thanks for your input!

[sunng87]

The latency may contain both data retrieving and heat map rendering.
It's a good idea to add indicator when heatcanvas is calculating. I will work on upstream leaflet plugin to add event listeners first.