Certainly it's possible for the script author to throttle. Not so much for the web site visitor.

I hope it's just a matter of time before browsers start to throttle sites...

Or maybe search rankings will be affected by CPU usage, and bandwidth.

Chrome has added throttling for inactive tabs. The problem is this is an active tab open for long stretches of time doing a somewhat cpu intensive task, so throttling isn't an option. I also doubt that be too adversely affected by slipping down the search rankings in this case.

Bandwidth and loads time already do play a part in search rankings, at least with Google.

Which is a lot easier toeasure than long running CPU usage unfortunately (at least not without privacy concerns - e.g. getting real world data from users)