I am sure you have heard once in your X10 life that you should use a decent task killer to kill apps running on background. After all, you have a 1GHz phone that behaves like a dumbphone (if not worse) in terms of speed. Some say killing apps saves battery and memory.
Don't be fooled!
It's not a memory issue, but a processor issue:
Its fine to have apps in background, as long as they don't waste your "processing power". To check how's your phone performance, i'd suggest apps like Watchdog Task Manager that show you hows every app using your phone's processor. That way you can spot the few apps that may have been slowing you down and wasting your battery juice and kill them.
You would notice that most of the apps running on background never use more than 0.5% of your phone. Killing more apps than needed would just slow you back down as you have to wait them to completely open every time you need them. Keeping them on background would cut the access time and save battery after all!
If you really have a battery issue, try lowering screen brightness, disable auto 2G/3G search (choose only one network instead). Know that Android, specially from 2.2 and later, have excellent memory management options, that way you shouldn't have problems using your phone the way it was meant to be. Try having your phone as up to date as possible (why not having Android 2.2 Froyo or 2.3 Gingerbread on??).
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