MacBook Air has delay when waking from sleep
Airs do go into standby mode more readily than other macs. 


MacBook Air (Late 2010) and later: About standby mode


Assuming its not just the extra time to exit standby mode (and isn't really sleeping anymore), have you checked the startup disk preference as the article above recommends?

You may have to look in the console log for sleep / standby messages in case your mac is entering standby / hibernate earlier than the expected one hour. The pmset command can show you the details of your Air's power management settings. Here's what a Mid 2011 Air looks like with mostly default settings:

mac:~ me$ pmset -g
Active Profiles:
Battery Power       -1
AC Power        -1*
Currently in use:
 standbydelay   4200
 standby    1
 womp       1
 halfdim    1
 panicrestart   157680000
 hibernatefile  /var/vm/sleepimage
 networkoversleep   0
 disksleep  10
 sleep      10
 hibernatemode  3
 ttyskeepawake  1
 displaysleep   10
 acwake     0
 lidwake    1


You can also use pmset to change either the number of seconds before standby kicks in, which is standbydelay; or turn standby off completely by setting standby to 0. Use the command man pmset for details.