I believe that _process
is simply a empty virtual function until you override it through GDScript or C#. I believe it is enabled by default, but since it is an empty function it should have very little performance impact, if any. There might be a small performance boost with it disabled, but I haven't had enough nodes in a project to know for sure.
I have not looked at the source code or anything, this is just based on my observations in Godot 3.0, Godot 3.1 and Godot 3.2 beta. The source code might operate differently.
(Side note: Welcome to the forums!)