Knowing what your system is doing at all times is a must :). Recently I was working and was seeing some usually slow system behaviour. Usually this is easy to spot, fire up your favourite task monitoring utility and do some digging.
I have recently been exploring tools on OS X and had learn to like MenuMeters. I was trying to use it recently and noticed is was reporting issues and thus not working.
I did a quick google and landed on the following link. Follow the instructions and you’ll have MenuMeters running on OS X EI Captain, again!