If you are having trouble with the frame rate when you display galaxies in Universe Sandbox:
Click on 'Settings' from the main menu and set the "Particles Multiplier" to a lower value. The default value is 1.0. Setting it to 0.5 will half the number of particles that are used to represent galaxies when you reload the simulation.
The setting only effects new galaxies, not ones that have already been created or loaded.
Let me know if you're still having trouble.