There is a perception that Windows Phone's multitasking is slow, despite the powerful nature of the Snapdragon S4 processors that power most devices, due to the "Resuming..." or "Loading..." screens that often pop up when switching back to a previously used app. Thanks to an in-depth post on Reddit, the perception of slow Windows Phone multitasking has been debunked.
Windows Phone has a number of states that an application can be in: Active, which is when an app is in the foreground; Deactivated, which is a background app that hasn't been killed and remains in the memory; and Tombstoned, which is a deactivated app that's no longer in the memory. Both Deactivated and Tombstoned apps appear in the multitasking list, and there's no way to visually distinguish between the two.
Apps also have a number of events that developers can use to trigger certain actions, including Launching (an app is loaded from scratch) and Activated (where either a Deactivated or Tombstoned app is activated). When you're launching an app for the first time, developers often have code run during the Launching event that delays the app from being fully loaded, although often a splash screen is shown and time-consuming actions aren't run.
Where the perception of slow multitasking comes from is from the Activated event. Developers don't check whether the app is Deactivated or Tombstoned when the Activated event occurs and so often make time consuming code run during the event, despite the fact that a Deactivated app can is resumed almost instantly with basically all functions intact, while a Tombstoned app takes a little longer and may require code to be run.
The perception of multitasking being slow comes from the apps being slow, and running unnecessary code when a Deactivated app is Activated. If developers coded a simple check into their apps when an app is Activated, a large portion of "Resuming..." screens would be prevented as extra code wouldn't be run as an app relaunches.