My EtreCheck Report - Poor Performance / Very Slow

This drive performance data stands out as a big issue:

Performance:

System Load: 1.19 (1 min ago) 1.17 (5 min ago) 1.05 (15 min ago)

Nominal I/O speed: 1.69 MB/s

File system: 24.11 seconds

Write speed: 64 MB/s

Read speed: 411 MB/s

The speed numbers should be more like these, from another 2015 iMac 27 with the same Fusion Drive configuration as yours:

Performance:

System Load: 0.92 (1 min ago) 1.71 (5 min ago) 1.27 (15 min ago)

Nominal I/O speed: 10.20 MB/s

File system: 20.97 seconds

Write speed: 520 MB/s

Read speed: 2005 MB/s

That is a huge difference. Something is very, very wrong with your Fusion system. It is not "split" or full, both common problems, so I have to suspect either imminent failure or a serious software conflict.

My suggestions start with running Disk Utility on your drive using the startup instrucitons here:

How to repair a Mac disk with Disk Utility - Apple Support

Then try these:

1) Disconnect any external drives and devices other than keyboard and mouse, and test. Basic standard action.

2) Uninstall or disable Sonos and test.

3) Try disabling Dashlane and testing.

Please see this User Tip for an external SSD setup: How to Setup and Use an External SSD as y… - Apple Community.

JSoove1966 wrote:

Thank you! I've completed the recommended actions, and the drive issue is becoming more apparent. It was actually flagged as a failing hard drive in the last report. I'll perform some additional backups and move to replace the hard drive.

Any recommendations for hard drives?

You would be best served by adding an external SSD drive, clone your current boot drive to it with Carbon Copy Cloner , and boot and run from the SSD. You'll get considerably more performance than you have and save the expensive cost of labor to install an internal SSD. Also when you move on to a new Mac you can take to the external SSD with you.

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:

Envoy Pro EX with USB-C or USB 3 - best SSD from a speed vs price standpoint

OWC Envoy Pro EX with Thunderbolt 3 - fastest (up to 2847 MB/s) if you have a Thunderbolt 3 port but more expensive

OWC Envoy Pro SX - up to 2847 MB/s  - also more expensive but fast

If you decide to go the external SSD route contact OWC's Customer Support to see which of their drivers would be the best fit, connection and speed wise, for your particular iMac model.

That is bad.

Problem One is that we cannot be sure whether the mech drive component of the SSD component is the culprit. Historically, it is the mech driv but not always.

Problem Two: is that the iMac case after 2011 was not designed for user access and I must recommend professional repair. Not all Apple Authorized Service Providers will work on these difficult Macs.

Adding One and Two you get a quandary: "Did I pay to replace the defective drive or the good one?"

Your computer should be supported for Apple service and parts until the end of May 2022, so I would definitely let Apple do the work. Peace of mind is a lovely thing.

Even the biggest US vendor of Mac upgrades and parts recommend professional installation: iMac 27 HDD