It's easier to solve a key if you know the public key, when someone spends from his address, public key associated with that address is revealed, this is what Satoshi did with puzzles every 5 key apart, methods such as kangaroo and BSGS can be used to speed up the search thousands of times faster than brute forcing for unexposed puzzles.

That's why you should never keep your coins on addresses with known public keys.😉 ( not that any time soon they can crack 256 bit keys, so every one is safe for a few decades )