iMac super slow, kernel_task hovering at 400 % CPU after upgrade to Sonoma

Your Fusion drive is broken. You can try to reconnect it using instructions here: How to fix a split Fusion Drive - Apple Support

This will erase all of your data. Now's the time to start that Time Machine backup.

However, it should be running better than that, even on just the one drive. I recommend a note to Santa asking for a new iMac.

iatethelotus wrote:

I logged in user a clean Admin account, and I don't encounter the issue of high kernel_task under it (after few hours of using it). [I don't even see the kernel_task process in Activity Monitor under this account.]

Can I conclude that the root cause is linked to something in my other user account?

Possibly.

But since this has been the subject of upgrades over upgrades for so many years (e.g. you still have Perian and Filp4mac installed even though they have been deprecated for almost a decade), I do think it is high time you make a good backup, actually make that two backups, and then erase the drive, install a clean system and migrate only the user accounts.

Luis, I had this problem and the culprit was heat. I found it difficult to work with external monitors or video calls without the kernel task taking >500%. To test this I put my computer on top of one of those large freezer cold packs meant for knee injuries. Problem went away immediately. So I had the machine opened and cleaned and the problem is completely gone. There was a great deal of dust in the fans. That's all it took.

One common reason for this high kernel task is heat.

If the fans are failing or the air vents are blocked, your mac cannot cool down efficiently.

In these circumstances, kernel task may "occupy" the processor in order to prevent other tasks from causing the cpu and other components to overheat.

Verify that the air circulation is not impeded. You may need to take your mac in for a cleaning.

You'd be surprised at the amount of dirt that can accumulate inside a computer.

You also have that piece of garbage more commonly known as "brickmymac". Sigh...

And in addition to all that: you have a load of old stuff installed on this mac, going back several years. DiVX? Really?

Do make a full backup, install a clean system (I'd do it in an external SSD and leave the Fusion Drive only for storage).

And if you do that, then migrate ONLY the user accounts. Make sure to get a fresh start, not encumbered by old and incompatible software or money-grabbing "cleaners".

iatethelotus wrote:

Talked too fast. Worked fine for like 20mins, then it was back to kernel_task close to 400 %CPU. Although this time my drives are fine, after I rebuilt the fusion drive.

Then you will need to take it in to Apple for additional diagnostics and repair.

There are two different procedures, depending on the mac models.

For macs with Apple Silicon, and for macs with Intel that have a T2 security chip, it is far easier.

Unfortunately, this does not seem to include the 2019 iMacs.

Start by reading this document and the ones it links to.

I believe I have written these instructions in detail some time ago, I will see if I can find them.

I am not exactly sure what you are saying, but if by "two drives" you mean "two volumes", like "Untitled 2" and "Untitled 2 - Data", that is CORRECT, and if you are erasing everything and installing again, you will AGAIN see those same two volumes. It has been this way since at least 10.15 Catalina. So if that is what you are doing, I am sorry but it wasn't the problem.