Lets just assume that you solve the "it takes a while to run" thing through some clever bits of hard-to-optimise math, that's difficult to parallelise or multit

Lets just assume that you solve the "it takes a while to run" thing through some clever bits of hard-to-optimise math, that's difficult to parallelise or multithread or whatever. If all it takes is computer time, then that's a cheap thing for the botnet operator to solve. They can just spin up another instance, and split up the crawling task to another computer (or 200).