[paranoidrobot]

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).

[Retr0id]

A captcha can always be parallelised. All the attacker has to do is attempt n different captchas in parallel.