Decreasing voxel size can increasing the solution run time exponentially to the power of 4 (assuming uniform part, like a cube). As an example if the initial voxel size took 5 minutes of run time, and then the voxel size was reduced by half the run time could approximately be expected to increase to 5*(1/0.5)^4 = 80 minutes. Currently voxel size is limited between 0.02mm and 2mm. However, due to memory requirements of smaller voxel size simulations this range of voxel size is not guaranteed to finish the simulation.