It took sometime to crack this one but I found a solution:
When you open a GUI with the guide command on a newly instold Matlab 2019 (and maybe 2018 as well) it sugest to migrate this GUI into the new technology App Designer. Even if you don't agree to do that Matlab, apparently, start a conversion calculation in the background. This occupies one of the computer cores and it is quite annoyin. While this is going on, you will see a message panel from App Designer.
To prevent this just unbox the 'Show App Designer message panel' in the Matlab GUIDE Preference. And thats it: no message and no background calculation.