I have had crazy high kernel task CPU usage levels with my Macbook Pro 15", 2015, fully maxed with 16 GB RAM.
I thought the following could be culprits:
- Using a 4k external screen and a cinema display for three total screens.
- Too many Chrome extensions.
- Using CrashPlan.
However, the problem might be none of these. I think I've tracked down the issue to Spotlight indexing my software development directories. So I've added any development related directories to the Spotlight exclude list.
This article describes how to prevent these folders from being indexed by Spotlight.
Given my heavy usage of RubyGems and NPM, and having zillions of development related files, this was likely the issue of my Mac becoming almost unusably slow.
I also like this tip of changing the default view for the Finder from "All Files" (which is insane for a software developer).
Here are a few more helpful links:
I also moved my Apple mail file over to an external drive. That saved about 50GB of space!