Tokinsom : oh, I see what you mean ! Thanks for clarifying Keep the sheet itself as a resource, instead of processing it and slicing it into frames directly on import. Indeed that makes resource management, especially animation updates, easier.
Though I think OP is mostly after building an animation from a spritesheet, regardless of the slicing, instead of manually importing one frame at a time, which is clunky and time consuming. Something C2 is capable of.
As for efficiency and performance, frames are packed into texture atlases when exporting anyway, so delaying the processing or not should be negligible.