Very fast?

It's been like 10 years they speak about that. It's like flying cars. Maybe it will never be a thing

Just google

In 2012 we were having a quantum computing revolution

https://www.google.com/amp/s/www.engadget.com/amp/2012-02-28-ibm-quantum-computing.html

To my understanding, the only concerns would be a bad actor being able to execute a 51% attack with minimal hardware or the sha 160 / sha 256 hashing algorithms being broken.

In the first case, it would be more profitable to any actor to continue mining blocks rather than perform double spends. So the likelihood is that quantum computing would make the network stronger rather than weaker.

In the second case, it was always expected that eventually these algorithms would be broken and replaced by something stronger just like they replaced earlier hashing algorithms. So we would be able to hard fork to a chain utilising new algorithms before this became an issue. Again meaning that quantum computing makes the network stronger, not weaker.