iPhone Simulator suddenly started running very slow
Solution 1:
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.
Solution 2:
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.