If it's an iPhone, then iOS will suspend background apps in favor of foreground apps. If your phone is lagging it's almost certainly because the foreground app

If it's an iPhone, then iOS will suspend background apps in favor of foreground apps. If your phone is lagging it's almost certainly because the foreground app is resource-intensive. If the foreground app requires more resources than are available due to background apps, it will suspend background apps to free up those resources. Apps don't really have much they can do to get around this, assuming you're not jailbreaking or installing local profiles. Well-built applications will have already saved state with every interaction. When they go to the background they will start unloading unnecessary resources and prepare for possible suspension by saving any additional data. Then when they're suspended, their state is saved to a snapshot that they can load in later so it's as though they never were suspended. There _is_ more capacity in recent versions of iOS for an app to use background resources, and to start a background process automatically. Still, the system treats background processes with a lower priority and terminates them for using too much power way more aggressively.