Delete partition of Fusion drive gives error

I had a second partition on the HDD of my iMac 2017/Fusion 1Tb. [It was for TM]

When I deleted the partition I got error messages.

At the end it seemed ‘successful’.

“warning: MT mapping (0x4000000bf7259 -> 0x1a38e0, 1, D) is not completely referenced

warning: MT mapping (0x4000000ce8689 -> 0x1cd1b6, 2, D) is not completely referenced

The volume /dev/disk0s2 appears to be OK.

Storage system check exit code is 0.

Operation successful.”

What is happening here? Is the Fusion drive compromised in any way? I know it is a common problem the Fusion Drive went awry in newer operating systems, and like many, I have blamed the new mapping that APFS forces in the drive (no central boot sector index anymore, all storage mapping being part of the file and hence distributed - a method an HDD cannot cope with say some experts.

But this makes me think the Fusion Drive is bad. Did an error creep in at some stage?

The Fusion drive is SLOW. Well after reinstalling Monterey: restart is 2 minutes but after a week it slows down to 3-5 minutes startup, and even wakeup is slow; launching programs takes ages too.

• Should I unmount the Fusion Drive and then create it again?

reason: my iMac goes to another user and I am cleaning it, installing Monterey and using TM of the new user.