I finally gave it another try today with the hope to fix the delay problem.
I did upgrade to 2.28.1 today.
The first thing I noticed was that after the startup was finished and I rejoined all devices by using them, the cpu load was again always higher then 15% and the used RAM > 439 MB.
I also recognized the CPU load slowly kept on raising.
Also I again had from the beginning a long delay. For example simple turning on a light in the Phoscon GUI took around 5 seconds to show it in the gui and around 5-15 seconds in real life.
My experience was this will get a lot worth if I keep it running for longer.
So I deleted all relays. This did not change anything.
Next I deleted all 7x _TZE200_bjawzodf temperature, humidity sensors and restarted the docker container because I still did not see any change in the CPU load.
After the restart I had 11% CPU usage and only 150 MB RAM. The light had only a 1second delay.
Reminder: Before on deconz:2.23.02 I had a CPU-load of 1-2% in general so I kept on trying:
Next I deleted the * 8x dimmer relays HK-LN-DIM-A and did another restart.
After a restart it had around 9% CPU-load and 137 MB RAM but the most important devices are missing now.
No big delays directly after the restart.
But it seems that the CPU load is linear to the amount of devices. The more I delete the more it goes done. Anyway I need all the devices and the system is to slow when all are added with all versions after 2.23.02.
So I keep asking myself, what did change after 2.23.02 so that the same setup takes 15-20 times more perfomance of my CPU and causes these delays?
What can we do next to debug this problem?
Now I wanted to return to my old setup which did work though… 
I imported my backup again from the phoscon gui and the gui was not reachable afterwards.
So I did restart the docker container. I was able to see my old configuration and devices but could not control the lights I deleted. So I did another restart. Same problem.
I deleted the new 2.28.1 container and reloaded the former 2.23.02 docker container.
I still can’t controll my 8x dimmer relays HK-LN-DIM-A. I looks like they lost the connection to the conbee II stick. After reconnecting them I can now control them via the GUI but not with the switches.
So if I press the light in the phoscon gui it toggles on off.
If I use the light switch it toggles in the gui but not in real life.
What is different now between a activation in the gui and a switch if the gui both time shows the light state change?
Now I remember why I did want to try to change this for a long time.
Never touch a working system. I will spend my time now trying to rebuild the former version so we can user our home again.
Any ideas how to procceed folks so me and @deadmedusa will be able to update in the future?