The global SPS performance is directly related to the global number of vertices.
The sphere mesh has quite many vertices as a SPS model. So multiplying 1 sphere by 8000 can make a really big final mesh.
I suggest that you reduce the sphere model number of vertices with {segments: 3} for instance, or that you use another model (a billboarded planar quad ?). https://www.babylonjs-playground.com/#ZEZLAT#47
If you just want to render points, the Point Cloud System could be a better choice than the SPS. https://doc.babylonjs.com/how_to/point_cloud_particles