Fraser Speirs posted a very necessary explanation about how iOS background applications work in an attempt to stop incorrect information from being spread.
It is well worth the read and I agree with almost all of his points, except the following statement that he makes a couple of times in the post:
The user never has to manage background tasks on iOS.
While this should be true, I, along with many of my friends and family with iOS devices, have not found this to be the case. If a user let’s the recently used app list grow uncontrolled, it noticeably slows down the device.
This is because an app in the recently used app list may or may not be consuming memory or CPU time (following the rules outlined by Speirs), but if an app is not in that list, guaranteed it is not consuming memory or CPU time. This means removing an app from the list is a method for forcefully freeing memory (or killing the app) and therefore speeding up the device.
As for why iOS noticeably slows down when it needs more memory, but seemingly doesn’t free up what it needs automatically, I don’t know what the reasoning behind that is. Perhaps the algorithm is not aggressive enough on freeing memory, trying too hard to leave apps alone?
The best example of this problem that I’ve seen was when I was in an Apple store and briefly tried out the demo 3Gs (running iOS 5) to find that it was barely usable; when I tried to type out a search on the spotlight screen, the keyboard “hung” with a key depressed for about 5 seconds and all animations were extremely jarring.
Surprised by how slow and unresponsive the phone was, I went into the recently used app list and cleared it out. Removing the first few apps was slow (taps felt unresponsive, animations jarring), but then everything sped up and felt back to normal. Going back to the Spotlight screen and typing again was a bit slower than on my iPhone 4, but very usable, and the animations were quite smooth now.
In addition, usually over once per day I’ve noticed the animations getting slightly choppy and apps becoming less responsive than normal on my iPhone 4. Clearing my recently used app list has always returned the speed and responsiveness that I’m used to with iOS.
I still recommend users clear out their recently used app list whenever they notice their device becoming sluggish as I know from experience that it works to speed up the device.