Android : How screenlocker applications open their activity instantly?

I'm writing an application which should display its main activity (there is already just 1 activity) instantly when everytime user turn on the screen. you can think it's like some kind of a screenlocker app. I can display the activity while the phone turns on however sometimes (not everytime) displaying the activity takes 1 or 2 seconds which is my problem. How can I prevent this waiting? How screenlocker apps can show their activity instantly? Is there a way out?