I have a Macbook Pro 13" Early 2015 3,1 GHz Intel Core i7, 16 GB, 500GB SSD, macOS Sierra 10.12.16
I was experiencing severe speed degradation, CPU overloading, the spinning beach ball and forced reboots for several months. It primarily occurred during videos playing in Chrome or Safari, and during video conference calling, but also with other applications. I was avoiding having to do a complete reinstall of macOS.
I went through the list of potential causes and fixes here:
The last thing I tried was turning off FileVault. It took about 1hr.
After turning off FileVault, the problem is fixed, and my Mac is back to normal speed!