Okay I think I have found the issue.
Only the left Thunderbolt 3 ports in my MacBook Pro supports the "looks as option" in the System Preferences.
Can anyone check if their MacBook Pro 13" shows the same behaviour?
According to the Apple website all Ports in the 13" MacBook Pro 2018 are full speed ports.
The 2017 had the right side ports with reduced data speeds..