Is this possible to improve boot time?
I do not know of a way of shortening the actual boot time: from power on to login dialog window. You might however be able to shorten your login time by removing items from your login items list.

You can find the list of user login items here: System Preferences → Accounts →  → Login items.

Remove stuff that has been added there that you do not want. Use some common sense though. E.g. if you use a calendar app that notifies you of appointments, it might be a bad idea to remove the calendar helper app from your list of login items.