Quantum computing would use Grover's algorithm, which is provably optimal for accelerating a brute-force search. That reduces the key space by half (so square root of the effort), eg AES-256 requires about 2^127 quantum operations to crack on average, instead of 2^255.

Bremermann's Limit[1] puts a fundamental limit on the rate of computation for any given amount of mass of about 1.36e50 bit changes/second/kg. Unless you get an amount of mass of literally planetary scale (as large as, say, Mercury) to take part in your computation the time will be enormous even for a 256-bit key.

[1] https://en.wikipedia.org/wiki/Bremermann%27s_limit