If I could choose in advance which kind of computations would be done, maybe. And there definitely should be an enforceable limit on the amount of processing.

It should not alter the user experience. Maybe use at most 30% CPU, leaving priority to the other processes. Also avoid computing on devices with battery, or with low battery.

Personally, if this processing infrastructure is managed by a big/trustworthy company (MS/Google/Amazon etc.) I should be OK with it, as long as its sandboxed. But if every individual website wants to implement its own way of using my processor power, I will prefer ads to it any day.