AS3 is fast as the language, but developers can make too CPU intensive animations there too. Moreover AS2 in which a lot of ads are still made is awful slow. And you can't control which ads (and the quality of the implementation) will be served when you're on some site -- therefore on weaker processors most of the sites which present the flash ads are amazingly hard to use -- you just experience lockups all the time.

> it's really quite fast.

Take a look at Silverlight. Its performance will surprise you, I am sure.