From your screenshots, your sim is very heavily reliant on PhysX. PhysX has multi-threading enabled by default, but internally PhysX threading efficiency is related to the number of "islands" that PhysX can split your particles into (islands are isolated groups of particles that can be simulated independently). Since your particles are in a giant pile, you're not going to get much speedup there because they're essentially a giant island that cannot be subdivided - that's why few threads are being utilized here.
In the PhysX settings you can enable CUDA (be sure you install the CUDA DLLs too) to see if that will offer a speedup. That speedup will rely on the power of your GPU for the rigidbody solve.
But in general, a simulation like this is tough to parallelize, simply because you have a huge number of interacting particles that PhysX can't internally separate into non-overlapping groups very easily.
In the PhysX settings you can enable CUDA (be sure you install the CUDA DLLs too) to see if that will offer a speedup. That speedup will rely on the power of your GPU for the rigidbody solve.
But in general, a simulation like this is tough to parallelize, simply because you have a huge number of interacting particles that PhysX can't internally separate into non-overlapping groups very easily.