iMac Fusion Drive performance concerns with macOS Monterey

Should You Upgrade Your Late 2015 iMac from Catalina to Monterey?

The short answer is that a Fusion Drive does not automatically mean your iMac will slow to a crawl after upgrading to Monterey. 

There is some truth behind the concerns you have  read, but the situation is not quite that simple.

Your iMac has a 4.0 GHz Quad-Core Intel Core i7 processor, 16 GB of RAM and a 1 TB Fusion Drive. That is still a capable configuration for everyday computing.

If it's currently running Catalina smoothly, there is ’no reason to assume Monterey will suddenly make it unusable.

1. Your Fusion Drive combines two types of storage:

• SSD (flash storage): Fast storage for frequently accessed files and applications.

• Traditional hard drive (HDD): Provides greater capacity but is considerably slower.

macOS manages both components automatically, moving frequently used data to the faster SSD.

The Fusion Drive is slower than a full SSD, but this limitation already exists with Catalina. Monterey does not automatically make a healthy Fusion Drive perform worse.

Some users experience slowdowns after upgrading because of background indexing, iCloud synchronization, limited free storage or an ageing mechanical hard drive.

The condition of your existing Fusion Drive is therefore more important than simply having Monterey installed.

2. Yes. Apple officially supports macOS Monterey on the 27-inch Retina 5K Late 2015 iMac (model iMac17,1).

Monterey is the newest officially supported major macOS release for this model.

Your processor and 16 GB of RAM are adequate. 

The main consideration is the performance and condition of the Fusion Drive.

1. I would recommend three simple checks:

A. Check the Fusion Drive

Open Applications >> Utilities >> Disk Utility.

Select View >> Show All Devices and run First Aid on the relevant volumes and containers. Look for reported errors or SMART warnings.

B. Go to Apple menu >> About This Mac >> Storage.

Ideally, have at least 100–150 GB of free space before upgrading. The are approximate numbers.

C. Create a complete Time Machine backup to an external drive before making any changes.

If your iMac already experiences frequent spinning beachballs, slow application launches or unexplained delays, investigate those problems before upgrading.

4. If you decide to install Monterey, consider using an external SSD connected through your iMac's USB 3.0 port as your startup disk.

This can provide:

• Faster startup times.

• Quicker application launches.

• Better overall responsiveness.

• Less dependence on the mechanical hard drive.

You do not need to open the iMac or replace its internal Fusion Drive.

A good external SSD can make a noticeable difference to everyday performance.

5. My recommendation

There is one important consideration: as of November 2026, Monterey is no longer receiving regular security updates. Its final release was macOS 12.7.6, released in July 2024.

Since Monterey is the newest officially supported macOS version for your iMac, I would consider the following:

1. If Catalina is running well, don't rush to upgrade.
2. Check the Fusion Drive's health.
3. If you need Monterey for application compatibility, consider installing it on an external SSD.
4. Always make a complete backup first.

I would not be particularly concerned about your iMac's processor or 16 GB of RAM. They are still reasonable specifications for everyday use.

The Fusion Drive is the potential bottleneck, but that does not mean it is necessarily a problem.

As noted, there are serious concerns give the history of Fusion Drives and the age of your iMac.

There is no real difference in performance between running Catalina and Monterey. The biggest problem this late in the game, is that the added stress of upgrading could cause a week or marginal drive to fail.

Either way, it is imperative that you have good backups in place. While using Time Machine on an external HDD or SSD can save your bacon. I also suggest using cloning software like CarbonCopyCloner or SuperDuper on a second external SSD. https://bombich.com/ https://www.shirt-pocket.com/

The other saving factor that is available for an aging iMac with a Fusion Drive, is the ability to startup and run from an external SSD. https://discussions.apple.com/docs/DOC-250003583

Welcome!

As other end users we obviously cannot make house calls to lay eyes on any issue. However in the case of iMac drive issues, there are a couple of free tools that can help us remotely assess your drive's performance.

One is Blackmagic Disk Speed Test in the Mac App store: https://apps.apple.com/us/app/blackmagic-disk-speed-test/id425264550?mt=12. Let it run for about a minute and then stop it and report the Write and Read speeds displayed.

EtreCheck does a more complete job, including a metric that is a relative measure of drive health. EtreCheck Pro is available here: https://etrecheck.com/en/welcome.html

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

👉🏻 IMPORTANT! Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

https://discussions.apple.com/docs/DOC-250000211

NOTE: Please post the entire report unedited. There are subtle hints there that may seem trivial to a new EtreCheck user, but can speak volumes to those of us who have evaluated thousands of them.

I've been reviewing Etrecheck drive reports for some time and have a decent idea of what speeds one should expect from a healthy iMac drive. A Fusion-equipped 27-inch 2015 should show Writes of 600-900MB/sec, and Reads 1400MB/sec and up.

The good news is that some known Fusion issues can be fixed at home by a novice user.

Thank you Owl-53 for the detailed reply. I'll check on some of these details you pointed out later this week. There's no big hurry on this because my wife uses the iMac for her quilting and sewing projects. I have my own Mac mini M-4 as our main computer.

Btw: I tried to give an up vote Helpful on your post but it says I reached my voting limit. I'll try again in a few days.

Thank you again Owl-53

ProTruckDriver wrote:

Thank you Owl-53 for the detailed reply. I'll check on some of these details you pointed out later this week. There's no big hurry on this because my wife uses the iMac for her quilting and sewing projects. I have my own Mac mini M-4 as our main computer.

Btw: I tried to give an up vote Helpful on your post but it says I reached my voting limit. I'll try again in a few days.

Thank you again Owl-53

You are welcome and thank you for the feedback

Just to compare notes I an using both the M4 Mac Mini and the M6 Mac Mini

The M4 is used most to Live Streaming to a 50" UHD 4k TV while the work horse for daily usage is the M6 machine

Agree, no rush and think things over at your leisure before moving forwards

🦉-53

den.thed wrote:

As noted, there are serious concerns give the history of Fusion Drives and the age of your iMac.

There is no real difference in performance between running Catalina and Monterey. The biggest problem this late in the game, is that the added stress of upgrading could cause a week or marginal drive to fail.

Either way, it is imperative that you have good backups in place. While using Time Machine on an external HDD or SSD can save your bacon. I also suggest using cloning software like CarbonCopyCloner or SuperDuper on a second external SSD. https://bombich.com/ https://www.shirt-pocket.com/

The other saving factor that is available for an aging iMac with a Fusion Drive, is the ability to startup and run from an external SSD. https://discussions.apple.com/docs/DOC-250003583

Good points my friend 👍

Allan Jones wrote:

Welcome!

As other end users we obviously cannot make house calls to lay eyes on any issue. However in the case of iMac drive issues, there are a couple of free tools that can help us remotely assess your drive's performance.

One is Blackmagic Disk Speed Test in the Mac App store: https://apps.apple.com/us/app/blackmagic-disk-speed-test/id425264550?mt=12. Let it run for about a minute and then stop it and report the Write and Read speeds displayed.

EtreCheck does a more complete job, including a metric that is a relative measure of drive health. EtreCheck Pro is available here: https://etrecheck.com/en/welcome.html

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

👉🏻 IMPORTANT! Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

https://discussions.apple.com/docs/DOC-250000211

NOTE: Please post the entire report unedited. There are subtle hints there that may seem trivial to a new EtreCheck user, but can speak volumes to those of us who have evaluated thousands of them.

I've been reviewing Etrecheck drive reports for some time and have a decent idea of what speeds one should expect from a healthy iMac drive. A Fusion-equipped 27-inch 2015 should show Writes of 600-900MB/sec, and Reads 1400MB/sec and up.

The good news is that some known Fusion issues can be fixed at home by a novice user.

+ 1 👍

ProTruckDriver wrote: Thank you den.thed for the added Good Points. 👍

You're welcome.

When I first posted, I didn't know that you also had an M4 Mac mini.

A few years ago I would have pushed you harder into the Monterey upgrade.

But today it really doesn't matter as much, because both 10.15 & 12 are out-dated.

Either way, she can keep running that older Intel software equally as well in either 10.15 or 12.

Plus given the fact that Safari is out-dated, you're probable already using FireFox or some other browser.

Yes I'm using the latest version of Firefox on the iMac. I might just leave the iMac alone since it's so old and I don't know how long the hard drive will last. That way I'll try to talk my wife into getting me an upgraded Mac mini and she can have this M4 Mac mini I'm on now. That might be a better plan.👍🤣

ProTruckDriver wrote:

Yes I'm using the latest version of Firefox on the iMac. I might just leave the iMac alone since it's so old and I don't know how long the hard drive will last.

I agree.

That way I'll try to talk my wife into getting me an upgraded Mac mini and she can have this M4 Mac mini I'm on now.

That is my normal Mode of Operation. 😇

That might be a better plan.👍🤣

Yea' you need to make sure that she has a reliable Mac to work on.

Thank you.

den.thed wrote:

Thank you.

👍

Thank you den.thed for the added Good Points. 👍