Very slow boot on Fusion and Big Sur

Here the result of a new test I did today. I copied to my iMac some big files from the network (about 8 GB). Then I installed Photoshop. After the installation, I restarted the iMac and the booting time jumped from the previous 19 seconds to about 1 minute. I started Photoshop and it took some minutes (including the verification). I closed Photoshop and I left the system idle for about 20 minutes. Then I restarted. That time the booting time came down to 30 seconds. I left the system idle for 45 minutes. Then I restarted. The booting time now was fully recovered to 19 seconds, as before the installation of Photoshop. Photoshop takes about 40 seconds to start. Word and Excel take about 13 seconds.

This test confirms that the Fusion Drive is now working as expected: when something is written, the SSD is always used. But when the system is idle, it moves back to the SSD all the blocks that are frequently used.

Just to remind that starting from Catalina, the Mac OS disk must be APFS formatted. Then I assume that all of us are using APFS ...

Concerning your last assumption, it's probably wrong. It seems the OS moves to SSD the blocks that are more frequently read, independently from the System or Data volume. Maybe the new 11.3 update changed some rules but, based on the results of my test (that show an increased boot time when large file are written on the Fusion Drive), my guess is that the system disk is not managed in a different way.

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)

Before I do anything more, I should know if what you mean by "low speed of (my) system" is the same as what I call "3 to 4 minute boot time". After boot and after waiting about one more minute (i.e. a total of 4 minutes from Apple boot logo to responsive apps), my system runs flawlessly and very fast. I never had a problem with Big Sur slowing down my system *after* boot. I only had and now have again boot times that are unacceptably long. So, are we talking about the same thing?

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?...

Is File Vault enabled on your system ? I'm asking this because with File Vault enabled usually the time elapsed between the power button and the login screen should be few seconds.

SAME "NEW" PROBLEM HERE: BIG SUR SURE IS A BIG LETDOWN FOR OWNERS OF IMAC WITH FUSION DRIVE!!!

On Mojave and Catalina, booting took less than 20 seconds versus at least a minute on Big Sur.

Now starts very slowly, with a hard stop halfway, a quick jumpy screen, then resumes loading... It's almost as long as it used to be on MacOS 9 (!?) Thank you Apple for ANY IMPROVMENT.

iMac (Retina 5K, 27-inch, Late 2015, 3.3 Quad i5, 16 GB RAM

Thank you, I should indeed try to muster enough courage to restart on Big Sur 11.1 from scratch.

My Fusion setup is actually the 2 TB, so its SSD size is 128 GB which may behave better than the 1 TB one with 24 GB SSD.

I used to be good at this kind of stuff, but the Apple Fusion logistics remains somewhat puzzling; any idea how to perform such clean install safely, so as to preserve the intended high speed and functionality?

Where did I chat?... I gave 5 logical, clearly technical points proving that you are wrong and I am right about what size of SSD Big Sur needs to not install itself on the HDD but on the SSD part of a 1 TB Fusion Drive in a less-than-one-year-old iMac. By stating that buying an external SSD to boot an almost brand new iMac is a "good solution", you are pushing people who are less computer-savvy than you and me, into making expenses for something that is totally unworthy of the design and the hardware of any iMac out there. It's not a solution to the problem, it's a work-around. And an expensive one, on top of that.

So, please, instead of trying to give me orders, prove me wrong on what I said in my five points above. If you can prove me wrong, I will gladly admit I was wrong. But until then, I have not read in your comments any logical, technical reasons why Big Sur makes so many iMacs with Fusion Drives boot up in 2 to 3 minutes whilst Catalina did that in 30 seconds.

Who is chatting here?

Your latest reply is not worthy of your 40 years of expertise. This is not a fight over who is wrong and who is right. It's a reasonable, logical, technical discussion or dialogue in order to find out what we, users, can do to make Big Sur boot in a reasonable time on our iMacs with 1TB Fusion Drives and small SSD blades, whilst hopefully Apple engineers, whom I wrote already three times since mid-November about this huge problem, are doing what they can to find a real solution to start the car's engine without having to pay some mechanics to give it a push (metaphor...).

Big Sur 11.2 is now released. Does it solve the slow boot for iMac 2017 1TB fusion drive machine? Besides, slow boot and getting "application not responding" messages, I also lost the internal speaker sometimes after updating to 11.1. Anyone with iMac 2017 1TB fusion drive machine tried the new Big Sur 11.2 update?

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?

Thanks for confirming this.

What do you mean: “more speed”? I just want my iMac to not run like it’s 1995. It would be weird to assume that the only way to accomplish this is by attaching an external disk. Something is very wrong with Big Sur, and I haven’t seen any solutions anywhere.

I don’t really see the point in starting yet another thread. I’ll just monitor the existing threads.

I tried that already, and as I did a clean install, the report returned nothing useful. I'll just have to accept that my iMac used to boot in 20 seconds and now takes 5 minutes. Guess I can leave it on 24/7.

Thanks for reporting back. Let’s hope they fix it soon. It’s very worrying that it took this long to at least acknowledge the issue, and even then it’s in a private call.

The fact there is no more uproar about this must mean it’s not affecting every machine with a fusion drive.

The engineer hinting that the issue might never be fixed, seems to be a real possibility. We’re already 5 months into Big Sur and Apple Silicon is the future getting all of Apple’s attention. If we want this fixed, it’s best to make sure the issue gets a lot attention.