Safari 'Slow Script' messages?

Asking primarily out of curiosity:

What is really going on with these "Slow Script -- do you want to stop
it?" messages that have begun appearing on web pages viewed in Safari?
Whose ox is being gored?   Whose needs are being served?

Are these scripts probably loading an ad, or driving some animation, and
Safari is giving us an option to cut them off?  (which means some
advertiser is losing what he paid for, and maybe getting mad at Safari).
Or what else are they likely to be?

How does Safari know that a script is "slow"?  In fact, are they "slow",
or just "long"?

The dialog box gives us no additional info to determine _what_ script
we're terminating, so there's really no informed decision involved.  So,
how about letting us just check a preference: "Allow slow scripts" or
"Terminate all slow scripts"?