Please run ShredIt Mobile for iOS in the foreground. Running it in the background will take a *very* long time even if you don't have a lot of free space. If speed is important, make sure to choose no government standards and only a single overwrite of a fixed pattern. Following these steps will give you the best performance experience with the ShredIt App.
ShredIt Mobile for iOS will write a large file to internal storage of your iOS device. This large file will fill up all the free space on your device and then it will be deleted to free up all the space again. You can configure ShredIt for iOS to perform this multiple times with different patterns. Because the file is unlinked when it is created, if something happens, like your device runs out of battery, the space is automatically returned by iOS.
ShredIt Mobile will report both the size of the file it has created and how long it takes to fill it up.