I agree with the general sentiment that rendering a million individual billboards is likely an overkill and the desired behaviour can be achieved in a more effective way. But I was wondering whether it’d be possible to do and worked on a Cesium plugin that does massive marker rendering. It works via points primitive and a shader sampling marker’s image, and it is more performant than Billboards.
Here’s a demo of rendering over a million markers around the globe: Cesium GPU Point Layer Demo
And this is a thread with some info on the plugin: Plugin for massive markers rendering & animation - cesium-gpu-points-layer
I hope someone would find it useful 