Why is my iMac (Retina 5K, 27-inch, Late 2015) running so slow?

1. Uninstall Google Chrome. There is a high correlation between slow macs and Chrome use. Go for Firefox, Brave, Vivaldi, etc.
2. Run Etrecheck or DriveDx. Slow performance with a clean install on 2015 hardware is usually drive related. DriveDx will look at the drive specifically, while Etrecheck includes a basic drive check as part of a full system diagnostic collection. Post the result using the "Additional Text" posting option if you have any questions.
3. Assuming the problem is your drive, reinstall MacOS onto an external USB 3.x SSD. Since you will be limited to USB 3.0 speed don't worry about top tier drives.

Doing yourself is not for the faint of heart. We read on these forums where do-it-yourselfers brick their Macs all the time.

IMHO the op should replace the computer due its advanced age and obsolescence but he’s determined to fix it.

I suspect he will be be back (or maybe not) complaining something won’t run any longer or some web app isn’t working. That is a common thing we run into with obsolete OSs and hardware.

Thats why I recommend cutting bait and moving forward.

A 10 year old iMac is really at the end of its service life and while we understand the emotional attachment the machine is obsolete and vintage. This means it cannot run current versions of Mac OS and will reach a point (sooner than later) where it cannot run the apps you want as the developers do not update older apps. The Vintage aspect means that if there is a hardware issue most parts are not available and an Apple Store will not be able to repair hardware. In short it’s time to face the reality of replacing it pretty soon!

I suspect the internal drive is failing, if it uses a Fusion Drive (combination of a very small SSD and a traditional HD fused by software) the HD portion is likely failing. The hardware test you ran is not very reliable and is rudimentary at best. Please first backup the system if you are not already, if you do not have have a backup your system is at HIGH risk of losing all of your data!!!!!! If you are not backing up get an external HD (www.macsales.com and get the Mercury Elite Pro series) and use Time Machine to backup, Time Machine is already installed on the computer . If you already use Time Machine great, please just test that it is working okay.

After you are sure the system is 100% backed up, download DriveDX and test the Macs Fusion Drive. Ensure you don’t just check the SSD it’s CRITICAL you test the HD as that is what I think is failing. If ANY errors appear on the HD than just verifies the HD is failing and you should replace the computer ASAP before it totally crashes.

Spending money on a 10 year old iMac, it's going to cost somewhere in the $300-$500 range is my hunch for a good quality internal SSD and the labor is silly. For about $1300 you can have a brand new (1TB SSD and 16GB RAM) Mac mini and a 27" 4K display that comes with a 1 year warranty, is eligible for AppleCare and will still run rings around your 10 year old machine. Your time and money of course but if that is what you want to do, okay.

Etrecheck says…

2TB Fusion Drive, nice, but running slower than expected.

CPU use a bit higher than expected. Looks like you have…

spotlight indexing still running. Try leaving your iMac powered on overnight give that a chance to finishes and see if performance improves.

DiskSpeedTest was open? If you had a drive speed test running at the same time as Etrecheck you should rerun Etrecheck without the competing test app also running.

You still have chrome or other google software installed as seen by the GoogleUpdater and Keystone agents. I really do recommend deleting/uninstalling Chrome.

If performance I will continue to recommend reinstalling onto an external USB SSD. Even at USB 3.0 speeds those are still about 3-4x faster than what your Fusion Drive showed in the last report and as an external drive there is first no added costs for professional installation and second the drive can easily be repurposed as extra storage if/when you later get a new Mac.

Thanks for the very useful response. I installed and ran Etrecheck and indeed the fusion drive is blown,

WAIT! Please POST the EtreCheck report. There are some Fusion issues that can be fixed at home to restore performance. Things in an EtreCheck report that may not stand out to a new user can speak volumes to those of us who have reviewed thousands of them. The developer made the app precisely for safe posing in these forums.

We can see hard data about drive performance, software issues/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.

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

How to use the Add Text Feature When Post… - Apple Community

Dom Hemeryck wrote:

I'm afraid, I'll leave that up to the experts. Too many imacs out there with cracked protective glass screens thanks to DIY'ers...

Indeed!

Have you found an "expert" who is willing to do this for you for a reasonable sum?

FYI, I have a 2008 24 inch iMac with an SSD and it runs really well on Linux Mint! 😉

Are you going to install an good sized SSD yourself?

I've watched some YouTube videos on how to do it. It doesn't appear too difficult! 😉

Sorry your EtreCheck attachment was non-existent. Please save the file to the Desktop and then attach it using the attach file icon in the “reply” window .