Fusion drive were good it there day but basically starting in Big Sur macOS 11 and newer version of macOS - the fusion drive will be the issue.
Big Sur and newer versions seem to designed around the idea of all SSD Drives. Yes, it will function on Fusion or Rotational drives but the performance will degrade massively.
There is a possible solution while waiting for the possible 27" M1 iMac to appear.
This would involve purchasing a Good Enclosure and SSD to be housed in the enclosure. The install Big Sur Directly to the External SSD Drive and use the External Drive as the Startup Disk.
The difference is performance will be massively increased.
In the meantime - one can run the below suggested Trouble Shoot Tool >>
Suggest downloading the Application Etrecheck directly from a well Respected ASC Contributor. And Safe to use.
The application is free or paid from added features.
Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).
It will take a Snap Shot - both the hardware and software.
The Report will Not Reveal Any Personal Information.
Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )
We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.
Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.
This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.
This will include CleanMyMac
This will include BitDefender
This will included Norton Antivirus
Sophos Av Software
Intego AntiVirus
The The Built in Security is all that is required.