Sure, they may not be that fast in constant-terms, but square root is still a *huge* speed up. Worse, that speed up gets bigger with more work, so reorging a few blocks at a time is even cheaper. The real question here is how many qbits do you actually have - it’s possible that we can change the PoW to SHA256x10000 instead of x2 and crank up the qbit requirement to make it impractical for quite some time.