Stop apps opening on their own when you log in
The queue of programs that launch with your Mac is editable, and most of what is in it got there without asking.
Tips tagged #performance
Watch CPU, memory and temperature from the menu bar
Activity Monitor tells you what is happening now; Stats keeps the numbers visible all the time.
Turn down the animations of macOS
Windows that stop zooming and desktops that stop sliding make the whole system feel quicker.