You can change how often the information in Activity Monitor is updated. By default, the information is updated every 5 seconds.
You can update information more frequently, for more precise monitoring.
Choose View > Update Frequency, then choose how often you want to update.
Increasing the update frequency may affect your overall system performance.
To see how much processor time is being used to update Activity Monitor information, watch the CPU column in the CPU pane of the Activity Monitor window as you change the update frequency.