All of the markers are merged into tiles and cached, and it is more efficent for the browser to just load these than it would be to create a separate circle or

All of the markers are merged into tiles and cached, and it is more efficent for the browser to just load these than it would be to create a separate circle or image for each marker. At zoom level 15 and higher actual individual markers are drawn (each one as a separate png image). There are still two layers, but both just contain image tiles: one contains the base map, and the other contains red circles on a transparent background.