"This intervention unloads ads that are in the .1% of bandwidth usage, .1% of CPU usage per minute, and .1% of overall CPU time. The current numbers are 4MB network and 60 seconds CPU, but may be changed as more data is available."
When this happens, Chrome will find the ad's iframe and unload the ad's content, and then use the same technology that's behind the Safe Browsing errors to show a custom message instead of the resource-intensive ad.
Tech news site 9to5Google, who first spotted the Heavy Ad Intervention feature, was able to reproduce this error message, which looks like the image below.
With advertising being a primary source of revenue for both Google and most of today's online websites, the company's engineers are now trying to restore a balance and a sense of health to this crucial ecosystem keeping most internet websites alive and kicking.