Federico Alzamora Previtali

Yes, these transient simulations can take quite a long time for a relatively fine mesh. As mentioned earlier, you want to have any boundary movement to be less than a cell width at any given time step to avoid collapsing cells. Depending on the objectives of your simulation and if the physics allow it, you could try to increase the size of your mesh and use a larger time step (at least for a first iteration of your simulation). 

Spring-based smoothing would be your best bet for this type of translational motion. Diffusion or Linearly Elastic will be more expensive and hence take up more CPU time.