First, you want to remove items from the startup items
Next, you want to remove unnecessary items from the following locations:
/Library/LaunchDaemons/
/Library/LaunchAgents/
~/Library/LaunchAgents/ (~ means your user's home folder, the full path will look like /Users/anyone/Library/LaunchAgents)
*.app/Library/LoginItems that you don't want this app to start (don't do this to MacOS's built-in app)
However, don't touch anything in the
/System/Library/LaunchDaemons/
/System/Library/LaunchAgents/
Highly not recommend to remove items of:
Your Anti-Virus software
Tuxera NTFS daemon
xquartz (if you want to use X server on Mac)