Creating world with multiple models

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I would suggest the following to try to improve the RTF without changing the physics parameters:

• Keep all your models simple, especially your collisions! I would avoid using custom meshes as collisions, instead consider one of the shapes provided by Gazebo (i.e. Box, Cylinder, or Sphere).
• If you have models not being used, maybe consider creating a plugin to turn off their physics or use kinematic updates when outside a specific range, and to turn it back on later. For example, you can try setting the model to static.