Performance drops when saving data from segmentation camera sensor

[adlarkin]

Environment

• OS Version: Ubuntu 18.04
• Source or binary build? Source: testing https://github.com/ignitionrobotics/ign-sensors/pull/133

Description

When using the segmentation camera sensor in #133 with the example world in https://github.com/ignitionrobotics/ign-gazebo/pull/853 (segmentation_camera.sdf), simulation performance noticeably decreases if I save segmentation data. I see an RTF of about 60% when saving images, and an RTF of about 97% without saving images.

Steps to reproduce

1. Build a workspace with #133 and https://github.com/ignitionrobotics/ign-gazebo/pull/853
2. run ign gazebo segmentation_camera.sdf, then press the "play" button to start simulation. The default sdf file saves segmentation data. Take note of the RTF
3. comment out the following lines from the SDF to skip saving data, and then run simulation again. You should see higher RTF:

• https://github.com/ignitionrobotics/ign-gazebo/blob/cc6a1919340bbc11b6ccd9ed4900f6f1e8e12d47/examples/worlds/segmentation_camera.sdf#L299-L301
• https://github.com/ignitionrobotics/ign-gazebo/blob/cc6a1919340bbc11b6ccd9ed4900f6f1e8e12d47/examples/worlds/segmentation_camera.sdf#L350-L352

Output