1) Asking a computer to do a computation is not such a great idea. Low powered devices (think iPads) running javascript would be at a great disadvantage to highly efficient botnet clusters that spammers own.
2) Requesting a small amount of money may work. Alternatively requesting a user to do some useful task (like Amazon Turk HIT) to get some funds.
Some computations just cannot be parallelized. (Yet. I'd be ironic if spammers advanced the field of parallel computing) The speed difference for single processors remain, but that's a single order of magnitude, except in extreme cases.
2) Requesting a small amount of money may work. Alternatively requesting a user to do some useful task (like Amazon Turk HIT) to get some funds.