Sequoia 15.6 slowing computer dramatically.

Third Party Disk Cleaners / Optimizer >> CleanMyMac , aka “ BrickMyMac “

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster. 

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a percentage on each sale. 

What the Application may do to the computer is up to the User to check this out before purchase

To put the Third Party Disk Cleaner / Optimizer in context and the damages it may have or has already done.

This type of applications can or will Muck Up your User Account ( Home Folder ) of this machine.

It does not touch the Operating System itself unless you consider your User Account ( Home Folder ) as part of the Operating System.

The Operating System is hosed

Once this type of software usinstalled and embedded into the computer 

Start Over from Scratch 

For Apple Silicon computer >> Use Disk Utility to erase a Mac with Apple silicon.

For Apple Intel computers >>   Use Disk Utility to erase an Intel-based Mac followed by How to reinstall macOS

Always make a Time Machine Backup  before  proceeding 

Emphasis on User Account Only

If going this route - I suggest Not using Startup Assist to migrate everything back.

This will probably Re-Introduce (CleanMyMac , aka “ BrickMyMac “  ) back into the Operating System 

blackjackstone wrote:

MacBook 16 is slow since updating Sequoia 15.6, Preview won't review jpeg on desktop...Spinning wheel.

Have used MacPaw Clean My Macx. Have reinstalled Sequoia. Anyone else

well I would uninstall your "MacPaw Clean My Macx" that is bound to cause more issues than resolve anything.

Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

all known to cause issues on the macOS

To trouble shoot further you can:

 —A SafeBoot Start up your Mac in safe mode - Apple Support will sort many anomalies

Does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, third party system modifications and system accelerations are disabled temporarily.

Login and test. Reboot  as normal and test.  Caches get rebuilt automatically.

This test will tell you if third party interference; most extensions etc are not loaded in safe boot mode. 

—Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support

  This will tell you if it a universal issue or isolated to your user/admin account. 

***wireless keyboard with rechargeable batteries— connect the charging cable, this will make the wired keyboard successful Safe Boot your Mac.

 unplug all non-essential  peripherals when testing

as always—Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

blackjackstone wrote:

Have used MacPaw Clean My Macx. Have reinstalled Sequoia.

Sometimes those pesky "cleaners" corrupt the system so badly that the only fix is to backup, restore factory defaults (or erase and do a clean install from Internet Recovery or from a bootable USB macOS installer) and check if it then works as expected. Then restore only user data files.

blackjackstone wrote:

Thanks. Turned out to be from an external drive being used for photography. Unhooked it, rebooted and appears to be back in order. Thanks.

Word to be wise 👍

Yes, unhooking the external drive for photography may have offered temporary relief

Though, be assured, at sometime in the future, the effects of CMM will rear its ugly head

It may not be today or tomorrow but it will and usually at a critical time