It’s not only about its Skeletal Mesh, it’s also about its collision, physic asset, anim blueprint, tick and maybe some other stuff I’m forgetting.
I think there are many “hidden things” happening when you set an actor to be hidden in game. Among these “hidden things”, I think Unreal does a bit of performance management and shut some things down such as animations. But I’m not sure about all the possibles performance leaks.
What I suggest would be adding 1000 your blueprint / SK to the scene, not hidden in game. See how much FPS you have. Now set them all hidden in game. See how much performance you have, if there’s no difference it means hidden in game is just about screen rendering and still calculates everything about the mesh. Now delete all skeletal mesh, run the game again, you should be at max fps, see if there’s any difference between 1000 invisible SK and 0 SK (obviously there will be). The aim is to profile 1000 hidden SK vs no SK and see what’s causing performance leaks.
When you found out, you can disable the thing cause performance leaks in the same time that you set actor hidden / visible in game.