macOS Sequoia 15.6.1 upgrade causes slow desktop loading and internet browsing

Do you have a Fusion Drive in your mac? If so it can be that the Fusion Drive is faulty

First try and boot your mac in the safe mode to see if the issues persist. if they don’t persist in safe mode the issues are most likely caused by an external piece of software, you might want to evaluate the software installed to your mac

Start up your Intel-based Mac in safe mode

1. Turn on or restart your Mac, then immediately press and hold the Shift key until you see the login window.
2. Log in to your Mac.
3. You might be asked to log in again.
4. On either the first or second login window, you should see “Safe Boot” in the menu bar.

To avoid a session of Q&A, Q&A  and Q&A  

Download the Application Etrecheck  ( External Link ) directly from the Developer.

This is a Diagnostic Tool that makes no changes to the computer.

It makes a coherent and readable inventory of both the Hardware and Software used on the computer 

The application is free or paid for added features. 

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - Share Report >> Copy , then  paste  >>>> using the Additional Text Icon  <<<<

iamshivam wrote:

You should also run Disk Utility First Aid to check the Fusion drive health, and if Fusion is indeed split, consider backing up and re-fusing the SSD and HDD.

Good idea in theory

Care to actually help OP, as to how to re-fuse a split Fusion Drive

That may really help OP with this possible issue

A Fusion drive is the pairing of a spinning Hard Disk Drive (HDD) with an SSD. The filesystem is spread across both disks and the SSD is used to boot the OS and run the Apps you use most often. Data that is accessed less frequently ends up on the HDD. If either the SSD or HDD experiences a hardware failure ALL DATA MAY BE LOST.

Fusion drives first appeared in entry level low cost iMacs. At the time, I had a Mac Pro and some folks figured out how to make their own Fusion drives. With the power of Terminal you had commands to setup an SSD & HDD to act like a Fusion drive. The only difference is it wasn't preconfigured by Apple at the factory. It ran pretty well for me but I had a higher-end SSD with more storage than what Apple typically delivered. i.e. they only had a 32GB / 64GB SSD doing all the highspeed caching paired with a 1 or 2TB HDD. The absolute lowest end iMac came only with an HDD. Again to reduce costs for the education market.

I really cannot stress enough, that you ought to replace the Fusion drive pair with a single SSD or two SSD's. Fusion drives have caused more heartache and pain over the years. Fusion drives should be put out to pasture. They are old and slow and unreliable. Ask at the Apple Store for an Apple Consultant who repairs iMacs. Many of these consultants have replaced many Fusion drives. They know how to take apart an iMac safely and how to change the drive(s) and get it all back together without breaking it.

But if you have an Intel iMac with a Fusion drive, it's likely time to get a new Mac. It may not run Tahoe 26 which is coming soon. Whatever version 27 will be called; it will only support Apple Silicon processors and Intel will no longer be supported.

You should also run Disk Utility First Aid to check the Fusion drive health, and if Fusion is indeed split, consider backing up and re-fusing the SSD and HDD.

+1 👍. 🦉