In-browser cryptocurrency mining is becoming a serious issue. While some sites like The Pirate Bay are open about the behavior, others (knowingly or otherwise) attempt to use your CPU cycles without your permission.
The behavior has reportedly drawn the attention of Google and according to Bleeping Computer, Chrome engineers are considering whether or not to take action.
Chrome engineer Ojan Vafai recently shared an opinion on the matter. In response to a bug report regarding resources being hijacked by in-browser mining applications, the engineer said that if a site uses more than XX percent CPU for more than YY seconds, the page should be put into a battery saver mode in which tasks are aggressively throttled. Critically, the mode would be accompanied by a pop-up notification that'd require the user to manually close (thus tipping them off that CPU usage had spiked).
Vafai said they'd obviously need to refine the XX and YY variables to figure out what works best but could start with egregious settings like 100 percent and 60 seconds.
It's worth noting that, to the best of our knowledge, this is little more than a suggestion from an engineer at this point and there's no evidence to indicate that Google is actively working on a notification or permission based on in-browser resource usage. Nobody likes pop-ups but I'd bet that people hate rogue cryptocurrency miners even more.
Until something does get done, your best bet may be to install an anti-mining extension like No Coin, minerBlock or AntiMiner.