Saving the current state? What does that even mean? You don't save transient things like visual particle positions but it's something you expect to persist between pauses.
replyPausing the render? But not the physics so you keep falling? You need to pause many systems. At the very least you'd want to pause gameplay, and sound and physics. You'd want to keep input and some of the UI unpaused. If you have a fancy UI with VFX, you need to make sure those are not using the paused game time. etc etc
And if you pause sound in Unity using AudioListener.pause = true, which is supposed to make life easier, but ends up being useless if changing settings/clicking buttons has audio feedback, or changing the volume has audible feedback to tell you how loud, or you allow to change voice style, and on and on.
replyRepeat that for every system - all those edge cases for each system can waste a lot of time and energy.
It’s only simple if your state machine has zero implicit state and all transitions are perfectly and precisely articulated. Good luck with that!
replyP.s. And once you are done achieving the above, you can then make sure you haven’t caused performance issues :)
But yes, conceptually, it’s a relatively simple idea. The devil is always in the details.