By default elementary OS is pretty slim, but there may be some applications you don’t use and in no need of running in the background. Example I don’t need orca or onboard to startup as I’m using a stationary computer.
WARNING: If you disable something vital you’ll end up with a broken system. Use with caution!
Open the terminal and copy/paste;
sudo sed -i 's/NoDisplay=true/NoDisplay=false/g' /etc/xdg/autostart/*.desktop gnome-session-properties
Now you can see what’s is set to running when starting elementary OS. Now be careful what you disable.
Tested on: elementary OS 5.1.4