When a battle is running and robocode is minimised the tooltip on the icon indicates that the battle is still being rendered, FPS is not 0. This can slow down the system a lot on systems such as the RaspberryPi which have very limited CPU, but otherwise run Robocode fine.
It would be better if no graphics are rendered at all when Robocode is minimised.
Has now been fixed for the coming version 1.7.4.3.