What might be happening with the Weather app is a trade-off between memory usage and performance. My theory is that in iOS7 the graphics and animation for each location were loaded when the app launched while in iOS8 these are only loaded when the user changes to a different location. So in exchange for lower memory footprint you get a slight judder the first time you switch to a different location. Given that most of the time people are only looking at their current location and the judder is very slight I think that the tradeoff makes a lot of sense.