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.