liggghts runs slowly

Hi CFDEMers/Liggghtsers,

My case is on the basis of multisphere_stone_restitution case. My question is that the number of my particle templates (multisphere) is 486, and the sphere particles is 21870 (nsperes=45). In fact, i need 30000 multispheres, that means the number of spheres is very large.But when i increase particle number ,the run is very slow, i think it is because the large number of particles .But something strange happened, the run does not keep a constant speed, even if a very slow speed, my case runs more and more slowly. I attached PDF file as below. Anyone give me a hint？

Thank you very much in advance.

wei-han