Slow real_time_factor with 6 arcucopters SITL

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Hello,

I am trying to simulate the behavior of a fleet of 6 drones by using ArduCopter SITL and gazebo 8.6. Each SITL is linked to a drone model using libArduCopterPlugin and libLiftDraftPlugin. My problem is the simulation is extremely slow : a real_time_factor of 0.23 in average, while my machine isn't doing much work : CPU at 10%, GPU at 40% and RAM at 25%. Is there a way to speed things up to achieve a real time simulation ?

My work is based on the iris.world example.

Thanks in advance.