Unfortunately, these performance monitors can make the computer use more power, by waking up every second to update the display, and therefore preventing the computer from entering deeper idle states. It's the same reason the default on-screen clocks no longer shows seconds by default, and blinking text cursors often blink for only five seconds or so before becoming static: waking up every second to update the screen can be costly.

Of course, a performance monitor which updates less often can avoid that issue (for instance, atop by default updates every ten seconds), but most people who run these graphical performance monitors want second-by-second updates.