Like any computer system, Android too has apps that automatically start on boot. Some of these, you probably don't want them slowing down your device's boot process, but for one reason or another, Google is yet to implement a menu that lets you choose which apps shall have these privileges, and which apps shouldn't bother.
Of course, the dev community is absolutely thrilled to present you with a solution by harnessing the Xposed framework's limitless power. If your device is rooted and you've got Xposed up and running, the only thing you need now is this set of easy to follow instructions:
How to prevent Android apps from starting on boot