iPhone Simulator suddenly started running very slow

I have been working on an app in iphone simulator for a number of weeks and it has been running well up until now, but all of a sudden has begun running very slow both when loading content and animations. I have not made any changes to my code since I last tested it successfully.

I tried restarting the simulator (multiple times) and removing the app and doing a completely clean rebuild, but no luck. I also checked my cpu usage through the monitor while the simulator is running and I am only using about 30% of my cpu and 40% of memory.

I fully understand that the simulator is never a quick as the device itself, but it seems strange that it has suddenly started running slow after such a long time, and by slow I mean less than a quarter of its original speed.

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":

Select Simulator,

Select Debug and uncheck slow animation.
shortcut command + t

That's work for me.

If the Cmd-T (slow animations) option doesn't work for you and Debug -> Slow Animations is off but you still have slow animations try Simulator -> Reset Contents and Settings (or possibly Hardware -> Erase All Content and Settings). That worked for me when none of the other answers in here did. Anyone have a suggestion as to why?

Also having a debugger attached (at all) may make the animations very slow.

Another potential fix for React-Native users:

Chrome de-prioritizes Javascript running in any tabs not in the foreground. So if you have enabled remote debugging, be sure to put the debugger in its own window.

You accidentally pressed the slow animation on debug when using simulator. So Run Simulator -> Debug -> Uncheck Slow Animation.

I think you pressed command + T instead of command + R by mistake.

I don't have the rep yet to leave a comment, but I upvoted some answers here and wanted to say more. I had a problem with slow animations in the iOS Simulator, especially on rotation, and I found this post via Google. Indeed, somehow "Toggle Slow Animations" must have been on, because three shifts fixed it. At first, I didn't think this was a problem because there's no checkmark next to "Toggle Slow Animations." It turns out there's never a checkmark, or any indication from the menu whether it's on or off. So just try toggling it and see if the rotation/navigation is faster/slower.

So, thank you!

Simulator -> Reset contents and settings works for me. The issue seems to reappear when I debug my react-native code remotely. It could also be to do with AsyncStorage as nomad suggested.

It is NOT only about slow animations. Xcode simulator has extremely low performance in global. It is Apple's bug. I have reported it via Feedback Assistant. I have created demo with code demonstrating that simulator is 200 times slower than any old real device. I have found that JavaScript code with Date object executed in WKWebView is pain for simulator. Changing options in simulator does not help in my case. See jsfiddle https://jsfiddle.net/kjms16cw/ I hope Apple will fix it soon!

<h3>Xcode Simulator Extremely Low Performance</h3>
<p>This test runs fast (several tens milliseconds e.g. 30 ms)

in any browser any device any platform including very old iOS device e.g. iPhone 5C and several years old iPad 2, BUT IN SIMULATOR IT TAKES 6000 ms (yes, 6 seconds!). Terrible!

run()

var log = document.getElementById("log"); document.getElementById("button").onclick = function() { run(); };

function run() {
	var d1 = new Date();
	for (var i = 0; i < 1000; i++) {
		var x = new Date();
		x.setMilliseconds(0);
		x.setSeconds(0);
		x.setMinutes(0);
	}
	var d2 = new Date();
	log.innerHTML = ((d2.getTime() - d1.getTime()) / 1000) + " seconds";
}

You can try to turn off remote debugging (Cmd-D -> Stop Remote JS Debugging). That usually speed things up.

When the chrome debug is active and the browser tab is not in the foreground, the emulator is slow too. For me, I decided to put the guide in the foreground.