The other day I had some background process taking up a lot of CPU while synchronising some data. I knew it would be slow and so I decide to read some news on the web. One of the pages I opened had some flash in it and after a few seconds a popup appeared asking if I wanted to kill that window. I assume that the background processes PLUS the greedy Flash player were making everything too slow.
It is interesting though that this is clearly a feature that appeared first in Chrome and apparently has propagated to Safari. A good feature, of course!