Is Gazebo running in just one core?

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Hello! I'm using Gazebo in a computer with the following hardware: Intel Core i7-4771 RAM :16 GB Graphics: Nvidia GT218 and it takes too long for the program to start, even ,my laptop is faster!! But checking in System Monitor, I get the following CPU usage graph:

It seems that Gazebo is just using 1 core at a time. Any idea why this happens?

Im using Ubuntu 16.04 and Gazebo 7