If you imagine that a laptop has enough hashrate to mine all the world's transactions in realtime, then how do you explain to yourself that the actual Bitcoin network is only able to add a few hundred transactions per second to the chain?
> The difficulty adjusts automatically. The energy usage comes only from the competition aspect - people are competing to earn blocks. You are lacking basic understanding of how PoW functions.
PoW works because miners have to invest significant resources in building and maintaining their mining rigs. As such, no one who wins the race to add a new block will want that block to contain false transactions, as that would bring down the value of the currency and leave them with extremely expensive mining equipment that is useless (once the deception would be noticed by others).
If the bitcoin network was run on 3 laptops, I could buy 100 laptops and start advertising my own blockchain which reverts the entire previous blockchain and replaces it with transactions I like. Since I can mine many more blocks with my 100 laptops, I can create a longer chain, so all clients will accept my chain and discard the old one, reverting all transactions that ever happened.
Instead, if I wanted to mount such an attack on the actual bitcoin network today, I would have to spend billions of dollars on equipment and electricity - ensuring that I wouldn't want to do so.