Not only are loading times and total network usage ridiculous, sites will continue to violate your privacy via trackers and waste your CPU even when background idling. I've written about these issues a few times in the last few years, so just sharing for those interested:

A comparison of CPU usage for idling popular webpages: https://ericra.com/writing/site_cpu.html

Regarding tracker domains on the New Yorker site: https://ericra.com/writing/tracker_new_yorker.html