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Same with me - just recently my iMac started to take 30 seconds to go to sleep. Checked it on the Terminal and indeed it's Chrome taking 30000ms before allowing it to go to sleep.

I have an iMac 2009 with Snow Leopard - given that others are experiencing this issue with Macbooks and Lion/ Mountain Lion, it looks like this is a fairly widespread issue.

I find this hugely annoying. Since there doesn't appear to be a solution, I've uninstalled Chrome and gone back to Safari. Without Chrome, I'm now back to the computer going to sleep almost instantaneously.
 
Update: it appears as though the recent update to Chrome (or Mavericks, who knows) has fixed the problem. Computer goes immediately to sleep even with Chrome running. Hooray!
 
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