When an UdonBehaviour is used fo e.g. some startup behaviour doing a few setup tasks, then sits idle forever, is it better to disable the component, or even destroy the whole GameObject, or does it not matter much for performance/memory?
There is some cost for each enabled UdonBehaviour in the scene. You could avoid this overhead by disabling the UdonBehaviour component or GameObject after it has finished all of its actions as long as you don’t need to have other UdonBehaviours access it. You could destroy it but that is probably not necessary.
To get a more reliable answer you will need to run your world in the editor and check its CPU usage using the Unity Profiler.