I’ve got 2012 Macbook Pro, maxed out with the fastest CPU (2.7 GHz), and it started to slow down, A LOT.
The Kernel Process would frequently take 400% or more of CPU.
I installed the Intel Power Gadget, which provides the CPU frequency.
What happened is that a dust build up inside of the case resulted in the motherboard overheating.
Incidentally, I spent a long time on the phone with Apple Support and their advice was I need to get my Mac serviced. They also had me clear all the application caches, which caused some minor settings to be lost.
The bottom line is that opening the case of your Macbook Pro and cleaning any dust could help A LOT.