Very slow boot on Fusion and Big Sur

Thank you, Tomeranaray, this is the kind of feedback that potentially helps a lot of people, who are willing to start from scratch with Big Sur. I wouldn't call it a solution as the procedure is far from what most users would be comfortable with, and a real solution would be that a live upgrade from a previous OS to Big Sur delivers the same result: a boot time around 30 seconds.

Just like you, some months ago, I did a clean install of 11.1, resulting in a boot time of less than 20 seconds but only before installing any apps like Pages, Numbers, Keynote, and iMovie. After installing just those, right from the App store, my boot times tripled, and adding Photoshop and Lightroom, once again went up to over 2 minutes. It seems that adding more apps and data "on top of" Big Sur made the OS decide it needed much longer and unacceptably long boot times.

I've got a few questions for you, related to this last remark of mine:

1. Do you have any third-party apps like Photoshop and Lightroom Classic or similar, and have you re-installed them already?
2. Did you re-install apps like Pages, Numbers, Keynote, iMovie, Garageband...?
3. What is the current size of your Music and Photos libraries?
4. How much free space do you now have on your Fusion Drive (it's a 1TB, isn't it?)?

Unfortunately, my 2019 27inch 5K iMac (i5, 3,1 GHz) with 40 GB of RAM and a 1TB Fusion Drive is still taking a full 2 minutes to get to the desktop and then I still have to wait another minute or so to be able to have apps start up quickly. Once 3 to 4 minutes have passed since the Apple startup logo appears on boot, everything runs smoothly and very fast. It's the boot time that is still absolutely abnormally long.

I have done the following:

1. A straightforward, in-line upgrade from Catalina to Big Sur 11.3, which gave no issues concerning potential loss of settings or data. Several bugs from previous versions of Big Sur seemed to have been ironed out. Boot times, however, remain a problem. Up to three minutes to have a responsive system.
2. A clean install, having "erased" all internal volumes and reset the Fusion Drive. When asked to migrate during Setup, I used a external drive which is a clone (a ChronoSync "mirror") of the non-system part of the Fusion Drive ("Macintosh HD - Data"). Alas: exact same results as above: same long boot time but no issues concerning settings, apps, or data - they were all nicely migrated.

What I haven't done yet, is a clean install, creating a new user account, and re-installing my apps and data from scratch. I'm not keen on trying that because the whole procedure is so time-consuming without any guarantee that it would yield normal boot times.

Apart from that, I have noticed something in the screenshot that Tomeranaray has posted with the results of the "diskutil list" command in Terminal. It could be meaningful in trying to pinpoint the cause of the problem on my system, but I doubt it for now. The SSD part of the Fusion Drive on his system is given the device ID "disk0", the HDD part is "disk1". On my system it's the opposite. When I ran the "diskutil resetFusion" command when starting procedure #2 above, the feedback in Terminal was actually correct and identified my "disk1" as the SSD part and "disk0" as the HDD part, saying the first was the fastest part of the Fusion Drive.

Could you, dear Fabio, run that command on your system, please, and post the result here, or simply give the respective device ID's of the two separate parts of your Fusion Drive?

Performance wise, I noticed three major issues, one going beyond the slow boot times:

• Time from pressing the power button to the login screen: the progress bar would get stuck for a while, or the screen would go black, causing major delays (>1min).
• The most obvious one is after logging into my user account, it took several minutes to get to the desktop (>3min). The screen went black with only the mouse visible, then the desktop appeared in bits and pieces while beach balling. Now, the background never goes black. I only have 1 account and the login screen adopts the same background as the account, so the dock nicely slides in from the bottom and within 10s everything is responsive. The difference is day and night.
• While I assumed my system was fast after everything booted up (after all, I worked on it for 6 months), it clearly was not. Apps are now far more responsive: Safari, Mail, Maps, all appear within 1 icon bounce, often without a single bounce. Even the first time after booting the system.

1. This is a thread about the incredibly slow *boot times* of Big Sur on Fusion Drive iMacs. On many of those iMacs, after a boot time of around 3 minutes or more, everything runs smoothly and very, very fast. As some of us have now seen their boot times back return to normal, it's obviously not due to hardware but to the OS itself.
2. I don't know how much "an extra $$$" exactly is in your country for an upgrade to a 2 TB SSD drive, but here in Europe it was an enormous amount of euros, about one third of the total price of the iMac.
3. When I needed to replace my old MacBook Pro one year ago, in 2020, in the country where I currently am on a mission since a few years, during the first lockdown and in a remote area like mine, it was not easy to get any iMac, without having to wait more than one month. I was lucky to find a 2019 27inch 5K 1TB Fusion Drive one, as not a single fully SSD iMac was available.

So, how could all this be my fault?...

Download and run Etrecheck. 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 include the report in your reply.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

Any solution that doesn’t involve buying external disks?

My 2015 iMac takes 5+ minutes to boot into a responsive desktop. Once it’s there, it’s working ok, but the boot proces is so painful. Even after logging into my user account, it takes minutes to build the desktop with the menu bar, dock, icons, all appearing over several minutes, screen going black, etc. This is Windows 95 all over again.

I did a clean install (without restoring backup) but that did not help. I noticed in Disk Utility my drive is split up as shown in the image, I guess is normal and caused by APFS?

I just upgraded to macOS version 11.3 and after logging in I got a brand new notification “Optimising your Mac”. It’s stuck on there for at least 5 minutes so I assume macOS is rearranging my fusion drive! Sounds like we might have a solution!

To rule out any software contribution to the increased boot time download and run Etrecheck. 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.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

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

Then we can examine the report and see if we can determine what might be causing the problem.

I can confirm that it is kernel_task in activity monitor that is doing the swaps between the SSD and HDD parts. My machine has now moved 1,6GB in the last 10 minutes. You can clearly see in the screenshot that the write/read ratio is almost 1:1. It's doing this while I'm actively using Safari. So there's no need to keep the machine idle for long periods.

(in case you have Time Machine active, it will list those data transfers separately under the backupd process)

You'd be best served by starting a new topic, describing your system and problems in detail and do the following:

Download and run Etrecheck. 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.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

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

Then we can examine the report and see if we can determine what might be causing the problem.

That way you'll get responses directed at your particular problem and not be lost in the bowels of this topic.

If you wanted performance, you would have purchased the iMac with an SSD, not that slow hybrid, rotational Fusion drive. If you were to perform a clean install of Big Sur on an external SSD, and change that to your default startup drive, you would see what I mean about the performance improvement. Once good source of SSD is OWC, or Crucial, both frequently recommended here.

USB 3, USB C or Thunderbolt external SSD all provide good performance. Below is a link to the one I'd recommend for a 2017 iMac if you want high performance.

https://eshop.macsales.com/shop/owc-envoy-pro-ex-thunderbolt-3

I'm experiencing the same issue and I agree. I reinstalled Big Sur twice without any improvement. After few reboots the boot time decreases, but it remains far higher than on Catalina. In addition, when writing a significant amount of data, the boot time increases again, probably because the boot files are removed from the SSD. Maybe Apple should find a way to keep "locked" into the SSD the most relevant system files and apps, in order to allow for a smooth processing.

It was the first thing I did, but it did not solve the issue. Maybe the size of Big Sur system files is greater than Catalina and everything cannot fit into the 24Gb SSD of my 1 TB Fusion Drive. After booting many times, the boot time slows down, but as soon as some large files are written, the boot time increases again. The solution would be to "lock" system files into the SSD. The 1 TB Fusion drive has only 24 GB SSD and under these conditions, it becomes almost unuseful.