Yeah, 20%CPU could probably have been improved in my latest implementation. Also note that CPU % varies a lot between computers, so maybe you have worse perf because you're previewing on browser vs NW.js or because you have a lower end laptop, I don't know.
As an effect, trail isn't possible as far as I know. C3's effect system doesn't allow us to define effects that get applied over multiple frames, and to work you'd need to specify a lot of parameters and it'd be too much work for an end result that wouldn't be insane.
As a behavior however, there is a point to be made, but the main limitation would be drawing on screen, and behaviors can't do that on their own.
The best solution IMO would be an object type with a ton of parameters that draws a trail over its last positions, and you'd just move it to wherever you want, and it'd draw a trail on its own. It'd be even more useful with the new Scene graph that's coming to C3, because then adding a trail would just be inserting the object as a child of another one.
However, IMO to get a great trail object, C3 would need two new object parameters: Graph param and color gradients.