Rebuilt Fusion drive is not using the SSD

My rebuilt 3TB fusion drive (on late 2012 iMac) is very slow (appears not to be using the SSD according to iStat Menus).

The 3TB HDD was replace by my local Apple approved service centre and configured to back to a fusion drive coupled with 128GB SSD. After initial setup which installed OS X High Sierra the boot times were 2 to 3 minutes. Using iStat Menus app I could see the SSD was not being used.

I have reinstalled the OS X many times via the recovery options, erasing the drive (formatting to Mac OS Extended - Journalled) installing High Sierra as well as erasing the drive and installing the original Mountain Lion OS X and then upgrading to High Sierra. I have read the the OS should automatically move files which are accessed frequently to the SSD but this doesn't appear to be happening.

In my frustration I have purchased a new machine with a 2TB fusion drive and compared the makeup of the fusion drives and apart from the different sizes (3TB and 2TB) they are the same (except the disk0s2 SSD protocol on the old iMac is SATA and on the new iMac it's PCI-Express), see attached System Info/Storage details.

I can't find any other info on this specific issue or anyone else having the same issue. I don't want to have to pull this fusion drive apart and rebuild it myself but that would be my next step. Am I missing something obvious. There was a post from someone who installed the OS on the SSD before creating the fusion drive.