Very slow boot on Fusion and Big Sur

I think your conclusions are correct: earlier versions of Big Sur had a problem where system files were incorrectly replaced by application files on the SSD, or some other SSD issue emerged causing the performance issues. On 11.3 I now have Office, Xcode, iWorks and some other apps installed with no performance degradations: a few seconds were added to the boot process and that's about it. I installed 11.3.1 yesterday and was quite nervous about it, thinking it would mess up my system again. I took very long to install (with at least 5-6 reboots) but performance stayed on par. I'm now quite confident the issues are fixed for good.

I had my problems fixed by a clean install of 11.3. (Before it took 4min to boot)

At the end of the upgrade process to 11.4, a message came my system would be optimised for performance. It took a fair while (reorganising files?) and boot times improved from 30s to 20s.

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.

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.

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.

Let's use some logic. After all, that's what software is based on.

1. Macs on Mojave or Catalina booted up in 10 to 20 seconds. Those Macs were already running things "what users install on their systems" (your words) or (in my words) 3rd party software. That software was not causing any trouble on Mojave nor on Catalina.
2. Upgrading from Mojave or Catalina makes those same Macs boot in 4 to 5 minutes - a problem that was already reported on the official Apple developers and beta forum last Summer.
3. It is as logical and correct to claim that Big Sur is the cause of the unbearably slow boot times as it is to claim that the cause is 3rd party software that was causing absolutely no problems under Mojave or Catalina. In fact, it is clear that Big Sur is INCOMPATIBLE with 1TB Fusion Drive Macs that feature an SSD blade of 24 or 32 GB. Big Sur is so "big" that it has no way of loading itself and the most essential apps from the SSD. It has to do that from the HDD part of the Fusion Drive.

exchanging the internal Fusion Drive to a 1 TB Samsung EVO SSD brought my unusable machine (iMac 27 late 2015“ back to 2021 on full speed and better performance than ever.

Forget an external solution.

I can definitely believe replacing the Fusion drive with a SSD drive will improve performance.

However, when Apple releases a new OS, performance should be similar (or slightly lower) than previous OS versions. Not like what a lot of us are seeing.

1. If the boot time of your system running once again on Sierra (or is it High Sierra?) is abnormally long, I suspect that during the upgrade - reinstall - downgrade etc. procedures, your Fusion Drive could have been split up in its two physical parts: the SSD blade and the HDD. Have you checked that? There are other threads on this forum that explain how you can check it.
2. I also suspect that replacing the HDD part of your Fusion Drive with an SSD, will make your iMac very, very fast, in boot and during normal use, even under Big Sur. As far as I understand the procedure by Apple, the original SSD blade will not be replaced and will actually keep its same function in combination with the new SSD, thus forming an SSD Fusion Drive. But this doesn't really matter to you as a user, it's a technical detail.
3. As a consumer you have consumer rights. If (a big IF) replacing the HDD with an SSD would not solve the problem, you don't have to pay anything. Just tell them that you want an absolute 100% guarantee that their solution will work, so that if it doesn't, you can get your money back. Especially because you already spent a lot of money on the RAM upgrade, which of course did not solve the problem...

Apple support or service people will tell you they will "replace the Fusion Drive with an SSD", but that is actually not exactly what they will do. Allow me to explain. The Fusion Drive is made up of two physical parts, the HDD and the SSD "blade", each of which are located in very different parts of the motherboard of the iMac. Upon disassembling the iMac the HDD is much easier to reach than the SSD blade (for which a lot of parts have to be disassembled to be able to reach it). As far as I understand the hardware of the Fusion Drive iMacs (you can check it in System Information), the SSD blade has a very fast connection to the motherboard: NVMExpress, which allows for speeds up to 8 GT/s - 8 GigaTransfers per second. The HDD part of the Fusion Drive is using a SATA connection, which "only" has a 6 Gigabit link speed.

Removing this SSD blade whilst replacing the HDD with an SSD would make your iMac faster, but not as fast as when they would leave the SSD blade in place whilst replacing the HDD with an SSD. Why? Because the "old" SSD blade (which has a much faster link speed than the new SSD that replaces the HDD) will still allow the new SSD to use it as a kind of extremely fast cache. In other words, the "old" Fusion Drive, made up of a "very fast" SSD blade plus a "rather slow" HDD, will be replaced by a "new" Fusion Drive, made up of the old, "very fast" SSD blade plus a new "fast" SSD (which only replaces the HDD, but not the old SSD blade). In still other words, the old, hybrid (SSD blade + HDD) Fusion Drive will be "replaced" by a new, uniquely SSD Fusion Drive (SSD blade + SSD drive), as the iMac will also "see" the combination of the SSD blade and the SSD drive as one "fused" drive.

I understand your frustrations, and I had them for 6 months. I was already planning to replace my late 2015 iMac and its measly 24GB SSD Fusion Drive with a new Apple Silicon machine later this year.

But now that I have done a clean install of macOS 11.3 I’m truly impressed with the performance of my machine. At this point I have the impression it’s even running better than Catalina. I fully realise that doing a clean install can be a hassle but so is buying a new machine out of frustration.

I also realise that doing a clean install may not be the solution for everyone and that my experience is no guarantee whatsoever it might work for you, but I do think that 11.3 has introduced some fixes. Our issue can’t be that widespread otherwise it would imply Apple has given up on machines with Fusion Drives, and that can’t be the case.

No, it does not. Some users report faster boot times (around 1m10s - 1m30s), but others, like me, see no improvement and have to deal with a 1-year old iMac crawling during 4 minutes or more until Finder or any app becomes responsive. It seems to me that the amount of space taken up by apps and data on the Fusion Drive, and the fact that Fusion Drives are by their very nature prone to defragmentation, both play a role in this. My 1TB Fusion Drive has more than 500GB of free space on it. It also seems that doing a clean install can - in some cases - either solve the problem or improve the boot times from 4min to 1 or 2min.

It‘s fixed for me now after replacing the internal Fusion Drive HD with a 1TB SSD. My iMac 27 5k Late 2015 is up to speed again. Cost 450.- EUR

PS. In yourt case, I would recommend asking for assistance - just in case things get hairy.

Some extra info:

https://support.apple.com/en-us/HT207584

About fixing fusion drive.

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 ?