Want to Close All Runnings Apps at Once

Question

is the only way to do this to either power down the ipad or close each one individually? When 30 apps are open at once...does this degrade performance on the ipad?

Answer 1

The MultiTasking Tray (name carried over from iPhone, and more than a bit misleading) contains the Recents List, meaning the ordered list of the last apps to have opened. Many people believe that all apps shown in that list are 'open and running'; this is not the case. The following is how I currently think that it works, and please keep in mine that I don't understand everything that I think I know about it.
Multitasking (MT) in iOS doesn't actually apply to most apps. 4.2 has a relatively small set (5-6?) of actions, mostly having to do with things like music playing and data being moved around or converted, which are allowed to continue when an app is closed. Most apps never touch those functions, and those that do won't necessarily be left in a 'running' state each and every time they're closed.
I know of no system limit on how many apps can be left 'running', i.e., in a MT state, but it will never be too many. iOS will leave apps open as long as possible but, as new apps are opened which need system resources now committed to MT, the OS will shut apps down in the order in which they were closed. Running apps can also be shut down manually, using the procedures outlined in the User's Guide to, essentially, edit the MT Tray.
The other main feature of iOS which comes into play when apps are closed is what you might call the 'state saver'. The OS uses and dedicates the exact same timing and resources to each app as it's shut: if the app can, and chooses to, do a state save at that point then what you will see next time you open that app is just what you were looking at when it closed. All you need to know about this is that not all apps will be doing this and the fact that one does doesn't necessarily mean that the app had been running.
So, is there anything to be gained by grinding through the MT Tray and 'shutting it all down'? Well, sure, I guess, sometimes. A little, maybe. But usually not that much. I personally don't bother, always have 40+ showing in the Recents list, and everything just keeps on truckin' on just fine. YMMV and, of course, everyone's gonna do what they want to do anyway.
I'm not really sure that the above applies equally to all apps anyway. For example, you can 'shut down' Mail, then start sending a bunch of messages to the iPad, all accounts. Notifications, at the very least, continue to work and all of the test messages are instantly there when Mail is next opened.
The issue of a 'mass delete' button for the Recents List might be a thing that some might aesthetically request; personally, I sort of think that if Apple thought you needed to be manually shutting down large numbers of apps on a regular basis then that feature would already be there.