Affect of Monterey to slow down computer

➡️ It appears your Time Machine backup is out-of-date. Make sure you get your backup running!

➡️ Delete Bitdefender following these steps: How to Delete Anti-Virus, Cleaners, VPNs,… - Apple Community.

➡️ Delete CCleaner following the same steps for Bitdefender. (above)

➡️ Delete the Anti-tracker Engine, Anti-tracker Icon, and TrafficLight Safari Web Extensions.

➡️ Other than that, the reason your iMac is very slow is due to the 5400 rpm hard drive. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Jack

I also have the late 2015 iMac and it has been upgraded to Monterey. The OS itself has not made it slow at all, though yes, in 2021 it is no longer considered fast especially when I saw another family member with their new M1 MacBook Pro.

Before I upgraded to Big Sur, and then Monterey, I took off my drive anything I didn't need and had about 30% of the space unused - of a 1TB drive. I found this helped keep the computer working nicely. It's still performing very acceptably.

The EtreCheck recommended by Jack-19 may diagnose some other cause of slowness but as I said, it's probably not the operating system.

Welcome to Apple Support Communities.

Let's first address why your iMac is slow. To do this, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

Omanni.orme wrote:

I do not intend to delete Bitdefender as I believe the additional internet security is necessary.

No one here can make you do anything but, no, it's not necessary. Apple has security built into the operating system.

Effective defenses against malware and ot… - Apple Community