It's more or less designed for it, it's SHA256 with a break in the middle for midstate compression to be effective, and the difficulty system is based on a misunderstanding of how bitcoin PoW works ("number of zeros" is never, ever a consideration in bitcoin, it's a match to a floating point target).

sha256(challenge + ascii(nonce)) means that the first compression round of the function can be cached and the second compression round is just the nonce plus the cache. This is the same trick used in Bitcoin mining and would have been avoidable by putting the nonce first, so immediately any non-naive code has to do half the proof of work as the vanilla solver.