2019 iMac 21.5" 4k runs painfully slowly

At the time of purchase you got the slowest iMac Apple offered: i5 CPU, 8 GB RAM and a Fusion drive with a 5400 rpm HDD. The read and write speed, Write speed: 259 MB/s and Read speed: 221 MB/s indicates a failing HDD.

As was suggested you could get an external SSD, clone you system to with with Carbon Copy Cloner and boot and run from the external drive. That would be less expensive than having an Apple Authorized Service Provider open the iMac and install a SSD replacement for the Fusion drive. And you can take the drive with you when you upgrade your Mac.

This user tip will tell you how to clone your iMac: Using Carbon Copy Cloner to Make a Bootable Clone of an Intel Mac - Apple Community

I strongly suggest you contact Customer Support at OWC (MacSales.com) and get their recommendation of the several option they offer based on your particular iMac model, your workflow and budget. OWC is considered by most here to be the premier 3rd party hardware supplier for Macs.

Just some food for thought.

the fusion SSD will show up in Profiler under NVMExpress (not SATA)

also noted in ABOUT THIS MAC> STORAGE tab

2019 iMacs are one of my things (I've upgraded numerous to M.2 and SATA SSD, and additional RAM)

The previously-recommended Thunderbolt3 ACASIS enclosure with the WD_BLUE NVMe M.2 SSD is unbeatable -- if you don't want to open your iMac and swap the fusion drives out -- the BLUE is the budget version (go with BLACK line if you want higher grade SSD) -- SAMSUNG MVMe SSDs like the 970 EVO Plus throttle down Write speeds 50% in external enclosures, but are my preferred SSD when internally mounted in 2017-2019 iMacs.

if your working boot system is good -- I would simply clone it over to the new external I recommended and reformat the fusion for storage (after you have a couple back ups on the shelf)

I would be sure your boot drive is NOT ENCRYPTED before starting the process -- if it is encrypted, I would start the UN-ENCRYPT process and give it a couple days to be sure it completes (it can take forever on HDD)

Download and run EtreCheck and save the report on your Desktop and then attach the report to your reply to this message.

https://etrecheck.com/index

We can then look to see if there are obvious issues with your iMac.

Your EtreCheck report's format is somewhat garbled and we may by missing something significant. All the section breaks are gone. Can you repost using the instructions in this excellent user tip:

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

As has been mentioned, from what the report, your Fusion drive system is severely underperforming. In your iMac model, expected speeds for a healthy Fusion configuration are Writes at 600-900MB/sec and Reads AT 1000-1500MB/sec respectively. Yours are:

Performance:

System Load: 1.72 (1 min ago) 1.98 (5 min ago) 1.68 (15 min ago)

Nominal I/O usage: 8.06 MB/s

File system: 31.41 seconds

Write speed: 259 MB/s ⚠️

Read speed: 221 MB/s ⚠️

However, a 31-second File system score suggests the drives are healthy. Although the post does not make it clear, you can try seeing if the components of the Fusion drive are "split," that is, not talking to each other. See:

How to fix a split Fusion Drive - Apple Support

using the diagnostic steps in the section labeled, "Before you begin."

➡️ Don't proceed beyond the diagnostic step until we have a chance to see what your found and have another look at the new EtreCheck report.

By the way, the "SM" in the SSD description suggests it is a Samsung SSD made for Apple, and the "HTS" in the mech drive description isimilarly suggests a made-for-Apple Hitachi mechanical drive.

Sir-Serf wrote:

I have four basic issues with this 2019 iMac 21.5" 4k.

1) It runs so slowly as to render it almost unusable.

If we are talking about a 21.5" iMac that is in working order, and that has at least 8 GB of RAM (as all 21.5" 2019 iMacs do), the most likely cause would be a slow startup drive.

The 1 TB hard drive and 1 TB Fusion Drive offerings for 21.5" 2019 iMacs were both slow. While the Fusion Drive combined a hard drive and some SSD storage, Apple got stingy with 1 TB Fusion Drives beginning in 2015. 1 TB Fusion Drives in 2019 iMacs only had 32 GB of SSD storage – prime real estate, but not nearly enough of it.

However, there may be other things going on with your Mac.

2) There is an issue with the App Store and Numbers, Keynote, Pages, and IMovie, where those programs insist on partially downloading themselves only to get hung up and not complete installation.

Possibly related to the volume corruption that Disk Utility reported.

3) It will not update to IOS 15.4.1. I have tried several times, and it always stops at "30 minutes remaining."

Macs run macOS, not iOS. Again, possibly related to the volume corruption.

Sir-Serf wrote:

Virtual Memory Information:

Physical RAM: 8 GB

Free RAM: 16 MB

Used RAM: 4.41 GB

Cached files: 3.58 GB

Available RAM: 3.59 GB

Swap Used: 24 MB

It looks like you had enough RAM for the workload that you were running at the time that you created the report. (You might get different results if you used Activity Monitor to monitor the Mac while you were running stuff that needed a lot of memory.)

Your Mac was using 4.41 GB of your 8 GB of RAM for macOS and applications.

Of the remaining 3.59 GB, only 16 MB (0.016 GB) was completely idle. The Mac put the rest of the RAM to use to hold cached data. It would have freed any of that 3.58 GB at the drop of a hat – but if there was a need to get the data, before there was a need to allocate more RAM, the cache would have saved the Mac from having to make a slow trip to the Fusion Drive to get the data. (In other words, it's a performance optimization trick.)

But there was 3.59 GB available. Swap Used was 24 MB, indicating that at some point your Mac ran out of RAM, and used space on the Fusion Drive to slowly simulate more. But 24 MB (0.024 GB) is a relatively small amount, so it doesn't look like you were doing so much swapping as to drag down performance a lot.

This just seems to confirm that the speed of the Fusion Drive is a bigger issue than the amount of RAM you have. And you can do something about the Fusion Drive (getting an external SSD) without cracking open the case.

How much space is left on your hard drive? This sounds like a hard drive failure, but I’ve seen similar issues from a disk with less than 20% remaining disk space.

Your drive holds 1 TB, and you only have about 350 GB worth of files. Throw in that 60 GB of purgeable space (assuming those are temporary files not included in the 350 GB total), and that's still only 410 GB.

So it doesn't look like you're seeing problems resulting from filling your disk to the brim.

The "Macintosh HD" label doesn't tell us what kind of drive your Mac has, since Apple uses that for all internal startup drives, regardless of type. The 1.03 TB capacity seems to show that your iMac has a 1 TB Fusion Drive; with the "extra" 0.03 TB corresponding to the 32 GB SSD.

That Fusion Drive could be slow even if there was no volume corruption and even if it turned out that there were no hardware failures.

You actually have my preferred machine iMac 2019 i5 — it will run Mojave and latest macOS sequoia

the problem 1 making it unusable is the fusion configuration — it’s 3percent fast ssd and 97percent slow 5400rpm spinning hdd

problem 2 would be the 8gb minimum ram

unless you can cut the screen assembly off flip the main board and upgrade the ram and fast (NVMe) ssd — or at least replace the SATA hdd with a SATA ssd

you are stuck

i recommend investing in an ACASIS 40Gbps M.2 NVMe SSD Enclosure, with Cooling Fan, TBU 405 Pro

and a Western Digital 1TB WD Blue SA510 SATA Internal Solid State Drive SSD - SATA III 6 Gb/s, M.2

then reinstall your system on the new ssd

that external enclosure will give you 2700 MBs sequential read/write through the iMac thunderbolt3 port

and that external will even be used with usb2 machines

Sir-Serf wrote:

One of them may have encryption turned on.

That would not have anything to do with your Mac. Encryption has been turned on and the machine is in the process of encrypting the data. That's going to bog things down quite a bit until it finishes.

Did you just recently turn on drive encryption? There are lines in the report that say "Fusion drive" / "Encrypting: 17% done".

If encryption is only partially done, the system is going to be crawling all over the entire drive, working on doing the encryption. That definitely could slow down anything else that wants to access the drive. Performance can take a hit when Spotlight decides to re-index an entire drive, for much the same reason.