It is difficult to comment on the working & efficiency of a quantum computer but the nightmare of every security expert is when computers come up with a solution for NP-Hard problems consequently deeming NP=P and compromising current encryption algorithms. Though NP problems won't become instantly solvable even with QC, they would reduce the time complexity of these NP problems and the cryptographic algorithms we use today.

There have been reports that Google's new quantum computer can solve 47 years of computing tasks in just 6 seconds. All we know is that they are exponentially faster and this is all without QC being still commercially used so we can only wonder how fast they would get.