Looked at their source code out of curiosity. They use Elasticsearch as a geo backend, tile server that renders PNG images server side for each map tile request.

- low zoom -> server aggregated grid

- high zoom -> switcher to point tiles

browser just displays images, there is no work to do on client side..

I generated a visual schematic how these kind of systems work: https://vectree.io/c/server-side-geo-tile-rendering-elastics...