Some of the developer options are handy and harmless (e.g. disable animations). However, there's one called "Don't Keep Activities" that you should not touch. This does not do what you think. It does not turn off programs/services when you exit from them in the GUI. What it actually does is purge them from the kernel's cache. Needless to say, this will make everything really slow (since it now has to be reread from flash memory ever time), and maybe even cause some crashes. Hurray for non-obvious config options! (And for me not looking stuff up!)
Turning the option off fixes the problem pronto. It's more just that one can set it and then forget about it, and wonder why Android is so slooooow.....
That option is for debugging/development purposes. http://developer.android.com/guide/developing/debugging/debugging-devtools.html Anyway, the developer options are hidden by default since Android 4.2. If I remember correctly, even if you did not turn off the tweaked setting, once you reboot it should revert to default.
