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After upgrading to Mountain Lion, now when I open the lid to my 2010 13" MacBook Air, there is a 3-5 second delay between when the screen powers on and when the keyboard will respond to key presses in the login dialog box.

This did not happen for me under Lion.

Is this a known bug?

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I am occasionally experiencing this too with my MacBook Pro (retina). –  jtbandes Jul 27 '12 at 4:30
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Please file this as a bug on bugreport.apple.com so Apple can fix it (if they haven’t already). –  Mathias Bynens Jul 28 '12 at 9:19
    
This bug has been enough to make me turn off the password protection on wake from sleep for my MacBook Air. –  Daniel Lawson Jul 30 '12 at 14:57
    
This actually happened to me under Lion as well, though it may have gotten more severe. Have you perhaps turned FileVault 2 on since upgrading? I believe Lion may have started exhibiting this problem once I turned it on... –  Dan J Jul 30 '12 at 16:12
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I can see the same thing with my MacAir (2012, 13"). What I noticed, as people have mentioned in their answers below, is: 1) It gets better if you restart. 2) It's usually worse after long sleep cycles (e.g. when I open it in the morning) –  GradGuy Aug 2 '12 at 17:03
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4 Answers

While I believe Macbooks Air did this before Mountain Lion, you seem to be experiencing a resume from standby. All Mac laptops, by default, write the contents of memory to disk when they sleep, but leave the power to the RAM on for a quick resume. Writing the contents of memory to the disk occurs so that you won't lose data if the system loses power.

However, on Macbooks Air and Retina Macbooks Pro, which have built-in Flash memory (which Apple distinguishes from 2.5" SSDs on Macbooks Pro), in order to advertise a 30-day standby time, after an hour the system removes power from the RAM. So, if you leave it asleep for more than an hour, you'll actually be resuming from the disk. It's a lot faster than on a device without an SSD, but it's still a noticeable delay. Apple calls this "standby", and while it's resuming you basically see a still image of the login screen that you can't interact with.

You can disable standby with this unsupported command: sudo pmset -a standby 0. This will cause your battery to drain much more when you leave the system asleep, but it'll make it resume much more quickly.

Apple's documentation on standby mode is here.

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+1 This accurately describes the issue as I experience it. It's worth understanding that this is probably not a bug and is simply an implementation detail (an unfortunate one: I'd rather have a "Resuming from standby..." screen with a progress indicator than a "frozen" login window I can't interact with!) –  Dan J Dec 6 '12 at 17:36
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I find that sleep/wake problems can behave somewhat better after a reboot. Try it, let's know if it helped.

Also, have you updated http://support.apple.com/kb/DL1559?

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As stated, I'm using a MacBook Air, not a MacBook Pro, so the above update is not applicable. –  Shawn Y Aug 1 '12 at 2:33
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After I freshly installed ML on my Macbook Pro Retina, I encounter the same 3-5 delay in keyboard-trackpad when the lid is opened and screen is on. Note that this usually happens after long sleeping hours. There is another thread that discusses the exact same problem here http://forums.macrumors.com/showthread.php?t=1415204.

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I reset the System Management Controller on my Macbook Air and the problem has gone away.

UPDATE: It came back the next day.

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