This is caused by a new feature called
Standby Mode which works on the MBP Retina and MBA. After an hour of sleep, the contents of the RAM are moved to the SSD so that the machine can power off more parts of the system to conserve battery power, the result being that your battery can last for 30 days in sleep mode. However, when you try to wake the machine up, the contents of the memory need to be copied back into RAM, which takes a few seconds. I'm guessing (speculation – not tested) that this might even be slower for people with more RAM (i.e. 16GB).
I doubt Apple can do anything about the wait time when coming out of Standby without changing the hardware, but I think the way the UI works at the moment is broken and hopefully they'll fix it soon with a ML update:
(1) First, they need to put in some kind of progress bar to show that the machine is coming out of Standby and that we should wait. The problem at the moment is that you see the normal login screen but it is frozen. You try to type your password but the keyboard doesn't respond. This just feels broken, so makes the problem feel worse.
(2) Maybe they should add a setting to System Preferences to disable Standby Mode. I can see this would be pretty useful for people who use their laptops all the time. I don't really care (most of the time) if my laptop can survive for 30 days without use, because I use it every day!
Maybe if we all submit feedback to Apple they will fix this bug.