acceleration of an inhomogeneous system

Hello everyone,
I have an inhomogeneous system that, there is a 30nm*30nm pva(Polyvinyl alcohol) flat floor at the bottom of the box and there are 8918 water molecules on the pva. What I am doing is to count the evaporated water molecules. And I will modify the pva floor to investigate the evaporation rate. The pva floor is fixed using fix setforce 0 0 0.
I did the test in my personal 4-core computer and the speed is 3.579 timestep/s. Then I submitted it to the HPC in my school where I can use 1 node of 24 cores. However, the speed is only 6.485 timestep/s. I know it is not possible to speed up about 6 times, but I was wondering what settings can I change to run it faster?
My guess is that, since I fix the pva floor, would it possible to put pva in, let’s say, 12 cores, and put water in other 12 cores? I look up processors and balance command, but not find a proper solution. Or is my guess wrong?
Could anyone give some instructions to speed it up?
Thanks a lot.