I have to disagree. I've been successfully using EFS to backtest and trade for years, and some of the new trading plugins and tools like trading directly from the DOM or chart are fantastic.
Maybe I don't use anything heavy enough to cause performance problems, but I'd have to think you would need to be using something very complicated for it to actually slow down the rest of the app. There were definitely some performance issues a year or 2 ago, but the last set of releases have gotten better and better as far as speed. Never even saw a hiccup on the last set of announcements.
One place to look and potentially ask some more specific questions is with some of the 3rd party developers that specialize in EFS. Chris Kryza is the first name that comes to mind.
Cheers,
Paul