What makes hash-based proof of work function is that to produce some number of hashes you need to use some amount of real-world resources. In practice, the two

What makes hash-based proof of work function is that to produce some number of hashes you need to use some amount of real-world resources. In practice, the two main resource inputs are electricity and computational hardware. If hardware is scarce, as it is now, the hardware will be a large proportion of mining cost, while the more efficiently hardware can be produced, the more of the cost is electricity.