From a privacy perspective, CPU load is clearly better than tracking. From a usability perspective, sluggishness from CPU load is arguably better than distraction from ads. There would also be less network load, which would especially benefit low-bandwidth users. And by the way, this would not involve persistent malware, and mining would end when users left websites.
Many have proposed micropayments as an alternative funding source for websites. I've often suggested cryptocurrency-based micropayments, with provisions for anonymity. With CPU-based mining, there's no added privacy loss. And it's a lot simpler, in that users don't need to buy tokens, manage accounts, etc.
It'd also be better from an equity perspective, I think. Even if micropayments for website use were low, they'd be a greater burden on people with less money. But everyone browsing has a requisite device. However, devices with weaker CPUs would need to work harder, and requisite mining might take longer, so there is some inequity.
What am I missing? Could this actually be a viable alternative to ads?
0) https://www.rt.com/news/404820-piratebay-showtime-cryptocurrency-mining/