13" Retina MacBook Pro - fans go crazy, computer slows down

Depending upon the specific model of you MBP, run an Apple Hardware Test ot Apple Diagnostics:

https://support.apple.com/en-us/HT201257

https://support.apple.com/en-us/HT202731

That may offer a clue.

Ciao.

"I ran all the diagnostics tests" ..

Might be an app that launches automatically when you startup your Mac which in turn is causing the fans to run high and slow the drive down.

Open System Preferences > Users & Groups then select the Login Items tab. Remove all items then restart to test.

And remove all items from /Library/StartupItems

Have you tried a Safe Boot?

This will tell us if something third party is causing the issue.

Usually when the fans are revving like this it is cause by a process that is eating up all of your computer's memory. The only way to check this is to open Activity Monitor and look for an activity (or activities) that are using up a lot of memory. Then quit the process.

Do you get a spinning beach ball when this happens? If you can have the activity monitor open to all process. It may give you a clue when this happens. As to weather it is related to an operation effecting memory cpu or hard drive.

Do you only get this thru CPU or is there also a momentary spike in memory as well?

When you notice the fan activity, note the exact time: hour, minute, second.

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console application in any one of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.

If you have an account on Pastebin, please don't select Private from the Paste Exposure menu on the page, because then no one but you will be able to see it.

Kernel Task is a combination of all the other tasks that run under root in the activity monitor. Since this could be because of one or maybe several root operations. Posting the log that Linc wants would be helpful.

What bluetooth devises are you using?