Throwing the ball out of the park, first of all, the Oxford English Dictionary states, LAG as falling behind in movement, progress or development. So people use this term when they find an action/task is performing slower time than what is ideally expected. Some examples of lag: 1. When opening apps on a 6 Plus, a short perceptible delay is seen which is absent on 6S Plus. 2. Clicking the home button twice on 6 Plus with iOS 9.0 produced a VERY perceptible delay absent on the newest iterations such as 9.2 betas. The word STUTTER has only recently come into use in describing software. A web definition states- "to speak in such a way that the rhythm is interrupted by repetitions, blocks or spasms, or prolongations of sounds" So in terms of software, its clearly used to describe animation behaviour (the way a software "speaks"). A stutter occurs when frames per second drops below the ideal mark of 60fps. They are more noticeable when it goes below 30fps. Now coming to the WHY IT MATTERS part, Drop in frame rates causes ugly & unwanted breaks in your workflow that hamper usability & productivity by taking away the focus. For some people it's actually a pretty big deal. And therefore it's paramount that the frame rate issues be fixed. Some examples of stutter: 1. Scroll on the list of cities on the weather app, first do a light scroll then a fast one, you would find a general lack of smoothness that's very unpleasant. 2. On the Air 2, with an opened page in safari, tap on the address bar, notice the animation where the URL goes into selected mode. There is very obvious frame drop, jaggedness in the animation. To summarize, lag is something where your action/task is delayed whereas stutter refers to the animation and it's lack of smoothness. Forum members are encouraged to give their own examples & explainations on this very prominent but largely ignored issue of stutter. Please file all your reports with Apple on bugreport.apple.com.