Which OS should I choose? I have a iMac (Retina 4K, 21.5-inch, 2017). Recently upgraded to Big Sur 11.5.2 and machine severely slowed down. Should I go back to previous version or forward to Monterey?

The newer OS versions all run better on a fast hard drive or SSD. If you have the standard hard drive (1 TB 5400 rpm), then it does not really matter which OS you choose - it will always be of snail like speed. The entry level 21.5" all had the slow hard drive. The only easy way to speed yours up is to make a bootable install/clone on an external SSD and then run your Mac from that, You can then use the internal drive as a Time Machine or other backup.

So, check your specs to see what you have and then you can make a decision.

GeoJoe1, not much mystery on this one. Take babowa's advice and set your iMac up to boot from an external SSD. I have exactly the same machine that you do. Mine went to a crawl just upgrading to Catalina. Now, (on Big Sur) booting from an external SSD it's almost as fast a my M1 MacBook Pro. I'm using the internal 5400 RPM HD for Time Machine and storing my application installs. I'm also giving Apple a good amount of time to get the kinks out of Monterey before I take the plunge.

Good luck!

I whole heartedly agree with Babowa on getting an external SSD to speed up your Mac. Unfortunately you bought the slowest iMac that Apple offered at the time of purchase: 8 GB RAM AND A 5400 rpm hard drive.

  Write speed: 34 MB/s

  Read speed: 61 MB/s

I recommend drives from OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties.  Consider one of these depending on your budget and needs:

1.0TB OWC Envoy Pro FX Thunderbolt 3 + USB-C Portable NVMe SSD

1.0TB OWC Envoy Pro SX Rugged Portable NVMe SSD with Thunderbolt/USB4

1.0TB OWC Envoy Pro Elektron USB-C portable NVMe SSD

Envoy Pro EX with USB-C or USB 3

OWC Envoy Pro SX - up to 2847 MB/s 

For your model contact OWC's customer support and get their recommendation for the SSD model that would best suit your iMac model and needs.  They have additional models other than those I've shown above.

I have the same year iMac as you but the 27" model. With an external SSD these are the read and write speeds that I get with Monterey:

If you go that route I recommend you reboot more frequently that you are, like shutdown at night and reboot in the morning:

Time since boot: About 2 days

My boot times with the external SSD are about 40-50 seconds so not inconvenient.

We should attempt to see what is slowing down your iMac with Big Sur.

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

➡️ Your Security and Updates are disabled. Please go to System Preferences > Software Update, and turn on the following:

• Check for Updates
• Download new updates when available
• Install App Updates from the App Store (optional)
• Install System data files and security updates

Then, go to System Preferences > Security & Privacy > General, and make sure the "Allow Apps downloaded from" is set to either "App Store" or "App Store and Identified Developers" (that may be the only options shown).

➡️ You have lots of Internet Plug-Ins. Go through and delete them in ~/Library/Internet Plug-Ins/ and /Library/Internet Plug-Ins.

➡️ I would recommend you use Safari, NOT Chrome. Google Chrome uses a lot more resources than Safari does. Plus, Safari is more private and faster.

➡️ 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.

Cheers,

Jack

You are backing up with CCC - good.

Why do you have security updates disabled - not good. Consider enabling them in System Preferences.

And, the report states (near the top) that the hard drive is slow.

As for your question re. your internal drive - DIY is not recommended and extremely difficult. Unless you can find an authorized service provider who will do it ($$) don't try it. A much easier solution is to get an external hard drive and use CCC to make a bootable clone on it. Then use it to run your Mac and use the slow internal for backups.

Good to have a couple of opinions, so do wait for Old Toad's and/or jack19's replies - they are very experienced.

+1 - Run EtreCheck.

This confirms that your iMac is slow due to the hard drive and not something else.

Jack

GeoJoe1, as for the external SSD selection, my opinion is this. The "internal" SSD selection is huge on Amazon and it ends up being significantly cheaper to purchase an internal SSD and then mount it (basically, just slides in) in an external enclosure. Not only does my iMac boot from an SSD configured this way, I have two more identical units that I have connected for backups.

MargeHomer wrote:

GeoJoe1, as for the external SSD selection, my opinion is this. The "internal" SSD selection is huge on Amazon and it ends up being significantly cheaper to purchase an internal SSD and then mount it (basically, just slides in) in an external enclosure. Not only does my iMac boot from an SSD configured this way, I have two more identical units that I have connected for backups.

Many cheaper SSDs are not high-quality, and I do not recommend using cheap enclosures.

Please see the specific recommendations from Old Toad and the User Tip.

Jack

I also only buy drives/SSDs from OWC - www.macsales.com

They are Mac-centric and have good customer support. There are no special features necessary: a hard drive/SSD does not need any software or driver to function. When you get it, plug it in and use Disk Utility to erase and format it. It'll be ready to go. Then use CCC to make a bootable clone on it from your internal. Test it to make sure it works and you can then boot into it and run your Mac from there.