On my MBP (mid 2011) the LED turns off and the screen goes on almost instantly. By almost I mean perhaps a split second, which is hardly noticeable. To make it come on literally instantly would be impossible due to hardware limitation. In other words, your screen isn't programmed to turn on with a delay, it turns on as quickly as it possibly can after receiving the wake-up signal.
As to what your problem concerns (assuming the delayed wake-up of your monitor is the issue), you should be able to start typing your password as soon as you wake up your macbook, so that when your monitor eventually comes on, you'll already be logged on.
Well on my MBP the screen goes on after 3 seconds. It's even faster when opening the MacBook while in Standby, but thank you for the easy idea of just start entering the password. This is an acceptable solution for me. – binarious – 2012-04-23T07:30:31.010