When you first launch a
brand new Mac its zips
through the startup
process.Then, as you continue to use it, startup slows down and seems to take much longer. As you use your Mac and install software on it, some of that software installs launch agents or launch daemons which load before login and thus slow down the startup process.
In addition, some apps install login items, which load after you have logged in, but before your Mac has finished starting up. Login Items are fairly easy to manage – see below. In general, apps or menu bar items that want to launch automatically when you login should ask permission when you install them. Whether you grant permission or not, you can change your mind at any time.