"A huge threat". Only to bad and mediocre encryption implementations If the encryption is well done, it can take trillions of centuries to crack it. And with a quantum computer it would take half that, because they only reduce the time by half. Even if the encryption if "not so good", e.g. with a passprhase that "only" takes 1 thousand years to crack, it would still take 500 years to crack it. Personally I'm not worried at all about quantum computers.
That's only true for symmetric algorithms. Public key stuff (which makes up most of the Internet), is vulnerable to quantum computing.
Hope this helps http://community.digitalmediaacademy.org/wp-content/uploads/2009/04/Asymmetry-Comparission.png This might be more helpful: "A problem with asymmetric encryption, however, is that it is slower than symmetric encryption. It requires far more processing power to both encrypt and decrypt the content of the message." https://support.microsoft.com/en-us/kb/246071 "Key sizes are not comparable between the two approaches. A 128-bit symmetric key may be equivalent in strength to a 3000 -bit public key. https://www.cs.utexas.edu/users/byoung/cs361/lecture44.pdf
Google has come up with a new quantum computing technique that could remove key limits on scalability today. -- Tom
Quantum Computation: A cryptography armageddon? http://www.welivesecurity.com/2016/06/14/quantum-computation-cryptography-armageddon/