I've installed Ubuntu 18.04 on a Virtual Machine (using VirtualBox). There I installed ROS Melodic (and Gazebo 9.0.0). I need now to perform a simulation of a simple robot in a simple world using an ad-hoc controller. However, I've noticed that, in general, my Ubuntu 18.04 VM is quite slow. I've also noticed that the FPS values of Gazebo are around 1-4, which I suppose it's quite low. In fact, my simulation is quite laggy and slow. What exactly do these FPS represent? And what are "workable" FPS values for Gazebo?