Improve GUI performance

[adlarkin]

Desired behavior

#736 results in good RTF numbers (90-100% on my machine) using TPE in a world of 3000 static shapes when running headless. However, running regular simulation (with the GUI) with the changes in #736 drops RTF to about 30%. There shouldn't be this big of a performance disparity between headless and GUI simulation. I'd like to figure out why using the GUI is causing such a noticeable performance drop.

Alternatives considered

N/A

Implementation suggestion

TBD - I will update this once I can figure out some reasons for the slowdown.

Additional context

None at the moment - I will update this if I find anything else related to this issue that is worth mentioning (could be related to #711).

It looks like the scene broadcaster system doesn't do much if there is no GUI: https:/ignitionrobotics/ign-gazebo/blob/ignition-gazebo5_5.0.0/src/systems/scene_broadcaster/SceneBroadcaster.cc#L255-L257

[chapulina]

It looks like the scene broadcaster system doesn't do much if there is no GUI:

Yeah I believe the SceneBroadcaster is the main reason for the slowdown when running with the GUI. See #556 for an idea that can shortcut the use of the SceneBroadcaster, by running server and client on the same process instead.

[chapulina]

Let's follow up in #556