This isn't even an actual issue, it is expected behaviour with "suspend to disk" (or deep sleep) and it is an annoying one. What it does is read the sleep image from disk and put all its contents back into memory. How fast it does this is entirely dependant on how fast it can read from disk and write to memory. Having an ssd helps tremendously in this case.
What happens with the Air in this case is that when you close the lid it will "suspend to disk" AND "suspend to RAM". I believe Apple calls it safe sleep but it is sometimes also known as smart sleep. This is useful when you swap the battery (which you can't do any more) or when the battery power drops to 0%. The Air apparently has a feature where after a certain period (someone here explained it already so read back; the feature is called "standby") the machine would save power by using the "suspend to disk" and stopping the "suspend to RAM". When you open the lid after that period the machine has to wake from "suspend to disk" which will be slow. Wake from "suspend to RAM" isn't, with an ssd it is almost instant (thus "instant on"). This behaviour seems to have been introduced in Lion and from what I see here it seems to be a bit worse in Mountain Lion (it takes longer).
The obvious way of solving the issue is by turning of the safe sleep function and only using "suspend to RAM". If you do that then be sure to not let the battery life drop to 0% If it's around 20% either sleep the machine and wake it when plugged into an outlet or charge it back up immediately. This should prevent power loss and thus data loss. Another one is to use 3rd party tools that will do "suspend to RAM" up until you reach a certain percentage of battery life. After that it will do the "suspend to RAM" and "suspend to disk" to prevent data loss should the machine run out of power. You can also change the behaviour of sleep to not go into "suspend to disk"-only after this safe sleep thing. This is done by setting the standby parameter of the pmset command (you need Terminal for this). This is what the pmset manual says:
STANDBY ARGUMENTS
standby causes kernel power management to automatically hibernate a machine after it has slept for a
specified time period. This saves power while asleep. This setting defaults to ON for supported hard-
ware. The setting standby will be visible in pmset -g if the feature is supported on this machine.
standby only works if hibernation is turned on to hibernatemode 3 or 25.
standbydelay specifies the delay, in seconds, before writing the hibernation image to disk and powering
off memory for Standby.
FYI: hibernatemode 3 means "suspend to disk" and "suspend to RAM" aka safe sleep (the naming is a bit confusing).