This is one of the reasons I still jailbreak my iDevices when possible -- I have a jailbreak tweak that shows me a list of all my actually running apps, not the most recently opened ones, and lets me close them all with a single taps. It usually shows about 5-6 apps as running. However, on my iPad 3. I don't think I've ever noticed any lags -- usually when I close my apps, it's because ONE app is misbehaving, but the way this tweak is organized, the "close all apps" button is easier to tap than to find and close the misbehaving app from the list.