the validation shouldn’t cause too much lag since game needs to sync up the game states anyways, which is an operation that is inherently way more expensive than any validation anyways (since each frame of the following game states need to adhere to the game rules anyways, there’s already inherently some form of validation). It’s more about not trusting everything the client says the game state should be.
And baldur’s gate 3 is one such game, it only runs the steam service (which includes a check that you actually can play the game either through ownership or family sharing) when steam is actually running so you can join multiplayer properly.