Grant Skinner recently published a highly interesting piece on unreasonable high idle CPU usage in AIR apps and the Flash Player.

Try this on a Mac: open FlashCS4. Create a new, completely empty FLA and publish it as an AIR file. Install it, run it, and examine it in Activity Monitor. On my brand new MacBook Pro, that completely empty AIR application uses nearly 4% of my CPU. Doing nothing. In the background.

Now look at the types of applications that are popular on AIR: Twitter apps, news apps, notification apps, mashups. Applications that are designed to be left open all the time. Open a few of these up at once, and you’re committing a decent chunk of your CPU to them. And that’s when they’re doing nothing. I typically have 10-20 applications open at all times. If these were all AIR applications, half of my CPU would be used just from having them open.

It’s something we really hope Adobe will take seriously and address in upcoming releases. Until then, as mentioned in the article, the problem can be partly remedied by simply reducing the framerate when idling in the background.

To some of us, this behavior is known from traditional SWF’s too – not only AIR apps.