From what i understand the chain is constructed in sort that there can be only one long sequence of operation for a given block height, so its not based on scanning a rangé of value with a statistical chance each round, but a deterministic set of rounds to solve the problem, and no partition can be computed in parrallel as each round dépend on the precedent one, it cant be made faster by breaking the work on different miners who can work in //.

I think the picture is wrong actually its not even based on hash.