iStat Mini for Mac gives you information about various systems on your computer, including Memory and CPU, at a glance right from the Notification Center. With this software in place, you only need to click over to the Notifications panel to see how your computer is working through a display updated in real time.
Lightweight: Because this app only provides a few vital stats, namely Memory Pressure, CPU, Disk Usage, and Network Activity, it doesn't take as long to gather the data or load as the full version. This ensures that your computer will still start up quickly while also keeping the stats you need at your fingertips at all times.
Constant updating: The fact that the app automatically updates itself in real time means that you never have to worry that the data you're looking at is accurate. You also don't need to wait for the display to refresh when you go to check it, saving you both time and frustration.
No details: There is no way to get any more detailed information through this app other than what is shown in the main display. For instance, if you want to know which apps are taking up the bulk of your resources at any given point, you'll have to invest in a more powerful software.
iStat Mini for Mac is a good option when you want access to some basic stats about your machine at a glance. It does have some limitations, but those are really by design, and they also ensure that the app does its job without slowing your computer down.