It's better performance for one web site, at the expense of whatever may be in other tabs or even outside the browser.

I really don't want to grant that.

Web browsers are failing badly at resource controls. I don't want to hand my whole computer over to a greedy web site. I expect my browser to stop the abusive web site resource consumption, not enable it.

While I agree that browsers generally currently appear to exercise poor control over resources like cpu time and memory amount, I'm not sure that aiming to cripple all sites equally is the right tactic here. It seems like better "scheduling" and other resource management would be something that needs work regardless of whether we expose tools to improve web application performance to websites.