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System Preferences > startup disk

Then click your mac OSX volume, this stopped it happening entirely for me.
 
Usually, I do not have a problem. However, it does occur every so often. Often enough for me to make a thread about it.

Do you have a lot of apps open when you put it to sleep? This may be why. I wish I had snow leopard to compare the instant on feature with
 
I had this same problem with my 2011 13" MBA i7/256/4 Lion 10.7.1

Someone on another forum suggested resetting PRAM, which I did. It seems to be working just fine now - wakes up almost instantly just like it should.
 
Do you have a lot of apps open when you put it to sleep? This may be why. I wish I had snow leopard to compare the instant on feature with
Chrome, Mail. That's all. I always wondered if it's Chrome. I'll see what quitting it before sleep will do.
 
Okay, so the next day after resetting the PRAM it started doing it again. I did a little more research and it appears to be an issue with the machine going into Hibernate Mode (not Sleep, but Hibernate).

Yet another suggestion by someone was to change Hibernate Mode to 0 so the system will not go into Hibernate mode, but will go into Sleep mode only.

I did try this and it seems to be working OK now.

I'll report back in a few days to see if the results are still the same.

Here's links for both PRAM reset and Hibernate Mode change:

PRAM: http://support.apple.com/kb/ht1379

Hibernate: http://www.tuaw.com/2011/08/22/why-hibernate-or-safe-sleep-mode-is-no-longer-necessary-in-os/
 
OK, so after several days now, changing the Hibernate mode to 0 has worked! 😀

It wakes up instantly now every single time - no delay at all...
 
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