i have a doubt why system take less time to start and take more time than start time to shutdown?
Because when you're starting up, your computer has to load drivers from disk, into RAM, using your CPU. It has to load the OS kernel, initialise and begin displaying pictures via your VDU, start sounding... sound, so on. When you're shutting down, it has to safely turn off devices, and save settings, and so, and... not much else!
So the reason it takes longer to turn on, is because it's doing more. A lot more.