2013 iMac formatted disk and now very slow

You did not describe what "disk 'cleaning' and OS restore" was done, and what macOS and what format (APFS or MacOS Extended) were used and what disk (HDD or SSD) is used. So more details are needed for an answer.

If the drive with the clean install of macOS has 900GB of Free storage space, then most likely the internal Hard Drive is failing. Many times a reinstall of macOS will finally push a worn out or failing Hard Drive over the edge since an OS install accesses many areas of the drive and runs the drive hard for an hour or so. It is not surprising that a Hard Drive from a 2013 computer would be worn out or failing.

Since you have a 2013 iMac, you can try installing macOS to an external USB3 SSD. This option will greatly improve system performance over a slow internal Hard Drive (or on par if the internal drive was a Fusion Drive). However, if the internal Hard Drive failure is severe, then it can impact system performance even when booting to external media.

As long as the internal Hard Drive failure is not extreme where it will cause problems even when booting to external media, then this

If that Mac has an old internal spinning HDD (not a newer SSD) drive and it is running Mojave or later with APFS format, it will be slow.

MacOS Extended with High Sierra will be faster, if that is the case.

Or maybe that Mac has a broken Fusion Drive?

You did not describe what disk (HDD or SSD or Fusion Drive) is used. So more details are needed for an answer.