Extremely slow SSD speed (iMac late 2015 27" 5K)

Your SSD:

Drives:

disk0 - SPCC Solid State Disk 512.11 GB (Solid State - TRIM: Yes)

Internal SATA 6 Gigabit Serial ATA

is SATA, not the faster NVME Express interface. Your max possible speeds are about 500MB/sec ±. Still fat better than what you have.

I suspect you have an SSD that is not fully Mac compatible. Best results seem to come with the Crucial MX (not "BX") series and OWC SSD/s I have a OWC 2.5-inch SATA 6 SSD in an older Macbook Pro and it happily does 500MB/sec.

Did the iMac originally have a Fusion drive.If so, you have a faster interface available.

Attila139 wrote:

I’m using a USB2 enclosure because for some reason I just couldn’t install macOS using a USB3 enclosure with a USB3 cable. I got the crossed circle screen every single time.

At a max of 480MB/sec burst, USB2 is far slower than even an under-spec SATA 3 mech hard drive on the internal interface. If the USB2 drive was the boot volume when you did the EtreCheck run, then the gawd-awful drive scores are probably due to that.

Did you copy or clone the old drive to the external? Cloning is NOT optional. I use Carbon Copy Cloner. I have no idea why a USB3 enclosure would error unless it were faulty.

Have you tried the USB3 on another computer? Did you format it as APFS?

Atilla139 wrote:

Installed macOS on an external SSD (it was even slower than with the internal SSD)

What is the external SSD and more importantly, what enclosure is it in & how is it connected to your iMac (USB2, USB3 (and if it's USB3 are you using a USB3 cable), Thunderbolt, Thunderbolt/Firewire, etc. ?

If both your internal AND external drives are giving equally poor performance it would not seem to indicate a problem with the drives themselves, but something that affects drive/system performance overall.

Belated noticed this:

• • Enabled TRIM

If TRIM has not been engaged, simply activating it will not cause an immediate change in speed. I learned this through experience with my Macbook Pro with aftermarket SSD.

Start in Safe Mode and let it "soak." I found I needed several safe boots left standing for 30 mins to 2 hrs to finally get my SSD back up to its original speed. The gives Safe Mode time to address drive issues.

But there is the "however."....

However, when not having TRIM engaged has allowed the drive to slow, it typically only effects the Write speeds. Both your Writes and Reads are severely depressed.

Still, the multiple SAFE Mode restarts should be tried if you only recent activated TRIM.

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

Usually the problem is found to be use to an installed app. You have a faster SSD, you have sufficient RAM installed. Do you have any antivirus apps or Mac cleaner apps installed? They will seriously slow down your computer and often cause other problems. They are completely unnecessary and should be removed.

It appears that you have a failing drive in the iMac:

  Write speed: 14 MB/s

  Read speed: 89 MB/s

That's worse than a 5400 rpm HDD. Make sure you have a full and current backup of your drive.

The read and write speeds should be in the neighborhood like mine are:

   Write speed: 1890 MB/s

    Read speed: 2401 MB/s

Whaat I would do is get an eternal SSD, clone your current internal boot drive to it and boot and run from the external. I get these type of speeds from a very basic external SSD setup:

which is far from being optimal.

My recommendation of to contact Customer Support at OWC (MacSales.com) and get their recommendation for which of their many offerings would be best for your particular Mac and budget. OWC is considered by many as the primer 3rd party hardware provider for Macs.

Also First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

PACE Anti-Piracy falls into this category. Uninstall it according to the developer's instructions. Additionally, your AdGuard ad blocking app is running in the background all the time. I would remove it also and use one that is only runs with Safari, i.e. AdBlock or Ghostery. Neither are resource hogs like AdGuard is.

Just to check, confirm that it is really booting up from the external drive ...

Those speeds (150 mb/s, 400 mb/s) are slower than HDD speeds and correspond to USB2, which is very slow. You should be getting 4000 Mb/s with an SSD connected via USB3. About 8mb/s equals 1 MB/s.