Google originally planned to start throttling background tabs in Chrome 56, but due to some concerns it would break a large amount of web pages, it was put off. Now the feature has finally arrived, as part of Chrome 57.
Here's how it works: every background tab has a time budget, in seconds, for running timers. The time budget goes into effect after the tab is in the background for 10 seconds. When a timer runs, the run time is subtracted from the budget. The time budget continuously regenerates, currently at a rate of 0.01 seconds per second, but that may be adjusted in future releases.
To prevent many pages from breaking, there are a few exceptions to this rule. Any tabs playing audio or containing real-time connections (like WebRTC and WebSockets) are not throttled. Google says that throttling leads to about 25% fewer busy background tabs, which obviously translates to better battery life on laptops.
To try this out, simply update Chrome on your computer or Android device to Chrome 57.