Bottleneck in real time factor

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Complex collisions (especially using mesh geometries), CPU ray sensors, and complex models (with a lot of joints and links) are some of the possible cause of slow simulation. Model and world plugins can also slow down the real-time factor if they are doing something heavy inside callback functions to physics-related events (world update begin, world update end, for example).

I found the Iris drone from this repo, not sure if it's the same one as you are running or not. A quick check doesn't see mesh collisions or CPU ray sensors, and the model doesn't look that complicated. The plugins may do something complex for calculating the drone's dynamics (I didn't check this). You can probably try to load the world without the plugins to see if it's still slow or not (very likely that it won't fly). If it's fast, then it's likely those plugins.

You can also reduce the <max_step_size> parameter to make it faster, but the accuracy of physics can go down.

Hope this helps.