iMac Unusably slow after High Sierra Upgrade

Possible Fixes for High Sierra

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Resetting your Mac’s PRAM and NVRAM
3. Reset the System Management Controller (SMC)
4. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
5. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
6. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button below Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
7. Download and Install 10.13.1 High Sierra Update
8. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
9. Erase and Install OS X Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Install OS X and click on the Continue button.

Ways to help make a slow Mac faster

1. 17 Reasons Why Your Mac Runs Slower Than it Should
2. Slow Mac Performance? This Article Solves It!
3. Fix slow start-ups in OS X | MacFixIt - CNET Reviews
4. How to fix slow shutdown and startup times. | MacTip.net
5. 6 Easy Tips to Speed Up OS X Yosemite on Your Mac
6. OS X El Capitan- If your Mac runs slowly
7. Visit The XLab FAQs and read the FAQ on Performance
8. Tips to Fix Issues that affect Mac Performance
9. Avoid using any third-party cleanup software. Usually, this software does more bad than good. You don't need it. All computers become slower over time even under normal use. Experienced users erase the hard drive and do a clean install from scratch from time to time; or whenever installing a major OS upgrade. Doing so means you must maintain regular and multiple backups.
10. If you have enabled iCloud Disk and are storing your Documents and Data in iCloud, then consider turning that off and signing out of iCloud. It can slow down the computer considerably. Please see the following from the user, fotomac: "The solution was to SIGN OUT of iCloud and my problem STOPPED! NO MORE SPINNING BEACHBALL! My computer's speed increased to what it should be and all my Apps now work!"
11. Add more RAM or cut back on the number of concurrently running applications and utilities. Remove unnecessary anti-malware software and any software that promises to clean your Mac. Check for runaway processes: Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity. Also, visit The XLab FAQs and read the FAQ on dealing with The Spinning Beach Ball of Death.
12. The Ultimate Fix: Backup everything, erase the drive, reinstall OS X, and restore your data from the backup. Reinstall third-party software from original media/scratch.

Before throwing the Mac out the window 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.

You can download Sierra from apple's site here: https://support.apple.com/en-us/HT208202

Just scroll down a little bit you will see it. You will need to download it to your machine. However since you have High Sierra installed it won't allow you to install it cause it's an older version. With that said what I did was use DiskMaker X http://diskmakerx.com/whats-this/ use the previous version which it has one for macOS Sierra, OS X El Capitan and Yosemite Compatible. Once you have downloaded Sierra to your system due not install or run it. Just go ahead and create a bootable USB drive with it using DiskMaker. After you can then go about reinstalling it to your system.

I have seen this issue crop up with fusion drives and it has to do with the new file format from what I can tell.

In addition the 3 TB fusion drives had a recall REP program from Apple so there may be that going on.

See apples site on the APFS. Prepare for APFS in macOS High Sierra - Apple Support

I did the EtreCheck (below) and would appreciate any help as to get my iMac back to normal/fast mode...

com.macpaw.CleanMyMac3.Scheduler.plist (MacPaw Inc. - installed 2018-05-18)

And BrickMyMac claims another victim.

Given that Etrecheck completed fairly fast (Performance: Good), the beach balling may indicate a failing hard drive. Most drives fail without any indication. Make sure you are backed up.

Back up your Mac, Boot into Internet Recovery, Erase the drive completely, and reinstall the OS. It's not worth anyone's time to try and track down what was damaged/removed/cleaned by the system mods you have installed.

You have also installed Western Digital's hard drive software. Completely useless and possibly harmful.

If you need to use those drives with Windows (not in BootCamp), then buy the commercial version of Paragon or Tuxera or just use ExFAT where you don't need third-party kernel extensions.

After reinstalling, during the Startup Assistant, Migrate only your user data.

Only reinstall the system modifications that are absolutely necessary--that's pretty much none, but you decide for yourself.

After erasing the Fusion Drive, it may detect that it is no longer Fused. If it asks, Let it rebuild the Fusion drive.

If it doesn't, you can rebuild it manually using these instructions: How to fix a split Fusion Drive - Apple Support

Sorry 8.4GB was a mistake on my part... remembered 4.8 as 8.4 :O

I wonder if the link down at Go to Mac App Store works different?

https://support.apple.com/macos/high-sierra

I mean it looks the same, but???

macOS High Sierra - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 14.3GB of available storage to perform upgrade*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

I haven't read all the way back on your problem... a new topic of your own may help, but...

Open Console in Applications>Utilities & see if there are any clues or repeating messages when running.

DanielDay wrote:

I noticed that iMac slowed down considerably after High Sierra upgrade. I thought something was wrong with my system, but I see other users are having same problems.

Yes, and perhaps thats true however the VAST majority of users have no problems at all.

Disconnect all peripherals and then retest, if the performance is still bad then it's likely a hardware problem. The ONLY reliable method of diagnoses is to take it to an Apple Authorized Service Provider that has the correct tools. It's possible one of your peripherals such as your NAS is causing the issue. This is why I am suggesting disconnecting all the peripherals to see what happens.

The NAS could be the issue too, Mac OS does not support NAS technology. Check with your NAS developers to see if the developers have updated their software to support the version of Mac OS installed. If they haven't then eliminate the NAS as that may also be the problem.

I wondered exactly the same thing. My Mac has never been the same since I did this 'upgrade' - they should term these things 'retrograde' as they inevitably ruin your computer. I almost lost all of my photos and had to reload them from the 'master' because this ridiculous 'upgrade' stopped me actually accessing Photos. If they have slowed the older Macs down, as they have for the older iPhones, they should come clean and tell us. I'm never - ever - doing another 'upgrade' and that's a fact.

I'm never - ever - doing another 'upgrade' and that's a fact.

Well, enjoy! Let us know how that's working for you in about three years.

What you and the OP have observed ins not systemic by any means. As rkaufmann87 has alredy pointed out, the majority of users have not such issues. I recently upgraded a five-year old MacBook Pro form 10.2 to 10.13 and it runs the same as it did before.

By far the most common cause of upgrade issues I've handled here--and I've been here a looong time--are due to Mac owners treating their Macs like Windows computers and loading them up with steaming piles of useless third-party utilities that falsely promise to protect, clean, and even paint the kitchen.