Increase Activity monitor update frequency to less than a second
I have found a few options:


Menumeters, says its incompatible with 10.11 due to code signing restrictions
XRG, an open source Mac resource monitor
Stater, $4.99, available on Mac App Store
Monity, $2.99, great if you want it in Notification Center
iStat Menus, $18, tends to be the most popular
System Monitor, $4.99, sits in menu bar.