Late 2015 iMac 17,1 having pausing issues

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I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

I see something way too familiar:

Drives:

disk0 - Mercury Electra 6G SSD 500.11 GB (Solid State - TRIM: No)

Internal SATA 6 Gigabit Serial ATA

disk0s1 - EFI (MS-DOS FAT32) [EFI] 210 MB

disk0s2 [APFS Container] 499.90 GB

I installed the same OWC SSD internally in my Mid 2012 MacBook Pro in late 2017. Last year its write speeds plummeted. Not as slow as yours but well under 100MB/sec for sure.

Per OWC recommendations, I did NOT enable TRIM when I installed the SSD in 2017. For three years the drive posted read/write scores very near 500MB/sec. Last year it started dropping, first to the 250MB/sec range and and, later, even farther. I saw lows on the 5-60 range, the same speed as the old mech HD the SSD replaced! Read was unaffected.

At the recommendation of the amazing Grant Benet-Alder here I enabled TRIM and did several Safe Boots. You need to let the computer "soak" in safe mode for an hour or more for all the sorting to get done. I had to do it several times. Even then, it was not back to speed immediately. However in a week or so it was back to its old scores. This is from a recent (24 July) test:

Happy again!

OWC says it is OK to enable trim. I referenced these OWC blog articles in changing the TRIM settings:

https://eshop.macsales.com/blog/?s=TRIMforce

To me, these values you saw are normal:

suggesting that connections and internal integrity are OK. Mine showed similar results when slow.

Offered for your consideration.

Allan

No Time Machine backup - Time Machine backup not found. Since your drive is possibly failing, please make a backup!

Here's the thing though; if it works for you, I wouldn't even try to open the Mac for a replacement SSD. Rather, I would just boot the computer from an external SSD and forgot about the failing internal. However, first, to confirm the drive is failing, please use DriveDx and upload the report here.

Jack

DriveDx report attached. It doesn't seem to be showing any errors.

That is quite odd. EtreCheck and DriveDx are not agreeing.

The Full Self-Test requires a purchase which I can get if that will help.

No need.

just using an externeral hard drive in its place. I don't disagree, but in the event that the hard drive is faulty, the drive is only over a year old, so still under full manufactures warranty - I'd at least like a refund or replacement to not feel like I threw money away for nothing...

I understand. However, it doesn't appear the drive itself is faulty, from DriveDx anyway.

If changing to an external hard drive, would the USB 3.0 connections be sufficiently fast enough or is there a Thunderbolt external drive to use?

Unfortunately, no one makes Thunderbolt 1/2 SSDs anymore, just Thunderbolt 3 and that isn't compatible with your Mac. USB is plenty fast, averaging ~420 MB/s read/write.

Just for testing, could you run EtreCheck shortly after a restart and post that report?

Jack

An iMac is not a user-serviceable product and we are generally discouraged to recommend that one open/disassemble an iMac to replace a faulty component. It can be a daunting task for many that do not possess the necessary skills and it is very easy to damage the iMac at any point of the process rendering it unusable.

That being said, if you've replaced the SSD yourself, then you already know the risks involved. I've performed countless upgrades/repairs on iMacs and MacBooks for many people. I was simply going to recommend to you that OWC is really good with supporting their products and as you have the chops to remove the motherboard (if you're using an M.2 SSD) or even easier if it's a SATA-based SSD (not requiring removal of any motherboards, then contact OWC first and they will most likely send you a replacement SSD quickly.

The reason I asked was because I'm not generally a fan of using an external SSD connected to USB. Don't get me wrong. It works and does the job. The issue is that when other USB devices are plugged in, the performance of the external SSD is impacted, especially if another USB device runs at a lower speed which then at times can force all speeds on the other USB ports to throttle down too. Again, it works. I had a suspicion it was you that did the SSD upgrade so here's my 2-cents. It's much more preferable to replace the possibly faulty drive. OWC will replace it, and if it's an m.2 drive, you'll get way better performance than the external SSD drive will provide. You do so at your own risk of course, but OWC obviously is doing well providing SSD drives for iMacs so someone is doing it.

It's why I still have my 2015 iMac. It's a great machine and will support MacOS Monterey.

at a guess I don't think it's had over 100GB on it.

Mine had under 130GB when it went south

Per the information Grant shared with me, Safe mode allows "trimming of unused blocks." Safe mode re-computes the list of blocks not in use and checks for cross-links. He did not say that the soaking is need, but I let it go a while to make sure all the the tasks associated with Safe mode were complete Try 30 minute to an hour, undertand you may e se ethe results immediately.

I have no idea why, after doing all this, it took a week or so for the results to show up. I have to take that as a mystery and be happy my OWC drive is once again speedy.

Who installed the OWC SSD in your 2015 iMac? Did you do it? I'll continue writing my 2-cents after you answer this question. I have a few thoughts as I too own a 2015 iMac.