I have found that most computers that are 4-8 years old are getting slow mostly because the hard drive is going bad. The hard drive starts taking a while to read or write anything, and you will start getting the beach ball all the time.
There are lots of hard drive tools out there, I found this one has a free trial:
https://binaryfruit.com/drivedx
But I have found even if the hard drive is on its last legs, sometimes the SMART system is really stupid and won't tell you anything is wrong!
The best thing to do is to upgrade the machine to an SSD, and the big SSD's are really cheap right now, you can get a teribyte for around $350, and a 256 is around $100.
If you have a macbook air or any newer thin laptop, it already has an SSD.
It is becoming a bad thing to click on something in your inbox...
This is a pretty nifty power monitor for your house for $300
Apple doesn't release a new app very often, this one is like a micro movie maker:
Vainglory is a lot like hero's of the storm, only its for your phone
Both are freemium
https://itunes.apple.com/us/app/vainglory/id671464704?mt=8