Hello ,about 3 weeks ago my old phone's sim reader broke so I decide to take my sister's old iphone 4s which she used for the last 4 years or so but about two weeks before she replaced it ,so I updated it to ios 8.4.1 ,I know it's an old phone but it ran it fine and smooth but yesterday after I downloaded an app which I deleted of course ,when I went into multitasking mode(double tap on home button) it started and is still lagging ,like I go into the multitasking and it takes like 6 seconds or more for me to be able to ,stop an app or even move around or lock the phone ,can somebody help me?