During booting message shows up "installer progress"

Try booting into safemode. Then change the settings so that there are NO wallpapers. Then reboot back to normal and test if that message is still there.

Start up your Mac in safe mode - Apple Support (English)

So I have finally resolved this.

1 Update all 3rd party programs including Firefox and chrome

2 Run a program like Clean my Mac or Dr Cleaner to remove all junk files and empty program caches

3 Get rid of any programs booting at startup

4 Reboot Mac about 12 x .

This seems to reorganise everything and the "installer progress" message clears and the iMac starts to return to the speeds encountered with previous OS

It took lots of rebooting to sort itself out but it got there in the end! I suspect this is the last upgrade Apple will do for Intel Macs and we are seeing the results of that migration to m1/2 chips. I am surprised that there has been no other mention of this problem from the technical support at Apple as it is a real issue as evidenced by both our experiences.

I also have a 2019 iMac and I have experienced the same issue. Also the computer is notably slower after this upgrade. I was hoping to go back to Ventura but the restore process from Time Machine backup won't work and I will have to go through the lengthy process of wiping the Fusion Drive and doing a factory reset and then restore everything with migration assistant. This is not for the faint hearted and I wonder why apple make this so difficult.

I have a Mini M2 and also have this problem, if that helps. I was going up to Sonoma

regards,

Dan