iPhone Simulator suddenly started running very slow
In the iOS simulator, at the bar on the top, click on Debug → Toggle Slow Animations (or Slow Animations with Xcode 10+). Chances are you accidentally toggled it on.
Simple Command + T will fix this problem.
Command + T toggles the simulator's Slow Animations, which can be found under the Simulator Menu: Debug -> Slow Animations.
Go to the simulator's Debug menu and select "Toggle Slow Animations".
Update: In Xcode 10, it's just "Slow Animations":
