Yes, thats a perfomance saver. The client doesn't verify blocks older than 24 hours. Think that the leeway of accepting blocks with bogus timestamps is +-2 hours, you cannot broadcast a invalid block that will get accepted by network for being too old to be verifyed, if you don't own at least 51% of the hashpower.
Anyone can verify the whole chain, it just that the stock clients does not verify old blocks because ECDSA verification is too computationally expensive.
So in short, you can be sure that blocks that skips verification because of being too old, has been verifyed by the majority of the network some time when the blocks was fresh.
So owning 51% of the network is always and have always been a "ownage" of the network and you can do whatever you want.