I've seen this guy using awt and Swing together. The game runs perfectly. I'm starting to suspect this is more of a urban legend.
Jframe is just the window border! How much do you think that will impact on performance?
I mean, having a good timer, good graphics and good engine is way better.
That said, this other guy uses JPanel instead of Canvas and the game runs equally smooth.
http://www.youtube.com/watch?v=qJpdRFvSj1A
So, I still don't know wich to prefer, in developing games.