Very slow boot on Fusion and Big Sur

Are you sure that the blade SSD of your 1TB Fusion Drive is only 24GB? I have a 2019 27inch iMac 5K with 1TB Fusion Drive and my blade (SSD) is 28 (not 24) GB. However, I do have the same problem: a boot time of around 2,5 to 3 minutes (from chime to the moment apps becomes responsive).

My guess is that the OP (jimdem582) has a larger SSD blade: 32 GB, as that would be the default size for it on a 2017 iMac 27inch with 1TB Fusion Drive.

Could you please check the size of your SSD blade again?

As jimdem582 reported: He managed to solve the problem with a clean install of Big Sur and a re-install (from scratch) of the user account(s), the apps, and copying back the data from a (non-Time Machine) backup. However, it could be (educated guess) that his 32 GB is just large enough to manage everything and anything that is needed to boot up normally and fast enough, but that a 28 GB (or 24 in your case?) one is not...

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

It solves the problem IF the SSD part of the Fusion Drive is sufficiently large and if there are no huge applications and large amounts of data (e.g. photo libraries with preview caches, music libraries of 100 GB or more) on the system. In my case, there is only a 28 GB SSD blade that speeds up the Fusion Drive. I re-installed everything from scratch, and right after installing Big Sur, the boot time was indeed something like 20 seconds, but as soon as I re-installed (not from Time Machine) a number of apps like Keynote, Pages, Numbers, and iMovie from the App Store, the boot time immediately increased to more than one minute. Having re-installed everything (and I do have a rather limited number of apps!), e.g. Photoshop and Lightroom, boot time was back to over 2,5 minutes, with apps becoming really responsive only after some 3 minutes.

The whole issue seems to lie in the size of the SSD part of the Fusion Drive. My iMac 2019 5K 1TB Fusion Drive has only a 28 GB SSD blade. Catalina was vastly smaller than Big Sur (8 GB installer versus 12 GB). Big Sur installed with a number of essential apps seems to be too big to make a boot time reasonably short.

Once the system is up and running after three minutes, all apps work fast and smooth. But if I want to shut down my system, I have to take into account that I better make a cup of coffee or tea when rebooting.

Nobody knows if 11.2 will solve this problem. I fear it's not going to.

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?

Probably you don't need to reinstall. Try to shutdown and reboot the Mac 3 - 4 times. After each boot, wait 15-20 minutes, in order to let the OS to arrange the data on the Fusion drive. If the boot time improves, you solved the issue. If not, try to reinstall. But after the installation, I suggest to do the same thing.

I tried reinstalling everything from scratch, as you described, and also creating a new user, in order to reset the profile, but nothing works. My boot time with Fusion Drive is about 1 minute 25 seconds. Probably the 24GB on SSD available on my 1 TB Fusion Drive are not enough for the Big Sur requirments (they were for Catalina). I solved booting from an external SSD on a USB 3.0 enclosure. Now it boots in 35 seconds and everything is very fast.

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...).

The system disk with the OS is 15GB. Photoshop is 3.2 GB. Final Cut is 3.6 GB. Word + Excel + Powerpoint are 5.5GB. It means that all the SSD space is filled up just by those few applications and every times a quite small video (1GB) is rendered, it kicks out from the SSD the blocks of some of the applications or of the OS. And next time the system boots or the application starts, it reads from the 5200 RPM HD.

I'm just wondering where is the "plenty of space" you were mentioning ...

My iMac is 5 years old and the external boot is a quite acceptable solution. But what about people who purchased a iMac with a 1TB Fusion Drive one year ago ?

Correct.

When I re-installed Big Sur from scratch without migrating any user account or restoring anything from a Time Machine or other backup, it booted my system in just over 20 seconds. Wow... But then I simply started installing Pages, Numbers, Keynote, and iMovie from the App Store, and the boot time became 40-45 seconds. After adding Photoshop and Lightroom, and restoring my Music and Photos libraries, boot time went up to 2m30s, often 3m before any app became really responsive.

The SSD part of the 1TB Fusion Drive in my 2019 iMac 5K with 40GB of RAM is a mere 28GB (not 32 nor 24), and the 1TB HDD spins at 7200rpm according to System Information.

How Apple is going to solve this problem without making Big Sur into Small Sur (i.e. reducing it to the size of Catalina) or allowing so many of us to trade in their Fusion Drive iMacs for a brand new SSD-based iMac, I don't know...

FWIW: When I run Disk Utility's Repair on the system drive after booting into Recovery Mode, I see a "warning" message (not an "error" but a "warning") reporting that a certain "keylocker" is missing: No such file or directory. When I run the same Repair after booting normally, there is no such warning. No other warnings or errors appear at anytime.

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?

I have a late 2014 Imac 5k with Fusion Drive. That particular old Fusion Drive has 128 GB of space - and the issue is the same. A LOT slower boot time than previous generations of MacOS. Furthermore, the drive is spinning audible under load constantly - that was not the case before.

I don't expect SSD-loading times. But I also don't expect new iterations of MacOS not support the Fusion Drive properly.

And that seems to be the case.

Hi. My old late 2014 iMac 5k has a 1 TB Fusion Drive - whereof 128 GB is SSD. Has exactly the same issues as described. So the 128GB SSD-partition is not enough.

Seems to be Big Sur not properly supporting Fusion Drives. Disappointing - although I don't expect proper fast SSD-loading speeds. Just something like on previous generations of MacOS.

Cheers,

T

Marathonianbull,

Had you read the Terms of Use that you agreed to you would know Apple is simply our host. They do not read or participate in user to user forums. The issue is not with Mac OS, frequently the cause is what users install on their systems.

Post an EtreCheck report of your system and someone will evaluate it so we can see where that issue may be.