The real_time_update_rate parameter determines the rate at which physics updates are taken. In conjunction with max_step_size parameter, the expected real-time factor can be determined. These parameters can be set via the gzclient gui or inside of world files (see friction_demo.world for an example).
To make the physics updates go as fast as possible, set the real_time_update_rate parameter to 0.0, and <joke>make sure the Turbo button is set to On</joke>.
An empty world goes over 200x real-time on my desktop. Inserting a double_pendulum_with_base model brings it down to about 20x.