At the scale an actual brain works, an actual exponential slowdown because of "quantum brains" would make the problem flat-out impossible. An actual human brain has an average of 100 billion neurons. To simulate an actual brain, you'd get an 'additional' slowdown of 2^100,000,000,000. The problem would go from possible in maybe 20-50 years to impossible unless we manage invent some sort of quantum/neural computer.

Not that quantum mechanisms in brains are likely, so probably not an issue.