Sweft's Law: The network hashrate of a proof of work cryptocurrency must rise at a rate at least equal to that of Moore's Law to provide minimal network security.
Moore's law is about the number of transistors on a processor. Processor speed is generally a byproduct, but hasn't kept up in percentage terms for quite a while now.
The security of Bitcoin's network is that it's always more profitable to play by the rules than to break them.