alt Hacker NewsIt's essentially the laws of physics. To oversimplify, Quantum computing can essentially do certain kinds of operations extremely fast (like factoring prime numbers) because it can calculate all the permutations almost instantly. But if you add intentional complexity to it in ways that all those states can't be "seen" then the quantum computer falls flat. That's one of the issues with adding post-quantum algorithms, they're by design less efficient in certain ways, meaning slower and/or with more overhead.
The way a quantum mechanics PhD explained it to me years ago in layman's terms is similar to nuclear science. We "knew" that a nuclear explosion was possible before a bomb was created and what conditions it would work. The Nagasaki bomb was a completely different type of bomb than the trinity test and Hirosima, plutonium instead of uranium, and it was never even tested before it's first use!
this is not an accurate description/heuristic of how quantum computing works. It would predict quantum computers can solve problems that they cannot solve. For a more accurate account see e.g.
https://www.quantamagazine.org/thirty-years-later-a-speed-bo...
And the post-quantum algorithms are not by design less efficient either. For example, RLWE-based schemes are more cycle-efficient than elliptic curve schemes. They're not uniformly more efficient (key/ciphertext sizes are generally longer), but this has nothing to do with intentional design choices to make them post-quantum secure. Just different things are different.