Interesting approach. I hope more browser vendors will adopt it. It comes with an additional advantage: if a website still needs to function when tracking domains are delayed a second or so, they likely will function too with said domain completely disabled (e.g. using extensions). Sounds like a win from a user perspective.
Trouble is the amount of popular websites that simply don't care how long the page takes to load, 30 seconds would be acceptable to many as long as they get their advertising dollars. Users won't know who to blame. They might blame the site but they're just as likely to blame Firefox, Microsoft, their ISP or a virus. There needs to be a display saying "hey, this site is slow because it's tracking you in 15 different…
Slow down pages -> more users bounce -> fewer ad impressions -> less ad revenue.