In Design Studio, default settings make the application remember the robot that were open when you closed the application, and open them again the next time you start it.
Because the number and size of the robots open at start can add more time to this process, please consider changing your settings by comparing starting times with the settings below.
Uncheck "Reopen projects on startup", in "Design Studio Settings" under the "Design Studio" tab.
This makes Design Studio not open the projects, and respectively their robots, that were open when the application closed last time.
With such configuration, the memory of previously open robots is cleared; so please make notes of them to be able to reopen them, and try this manipulation.
Below is a screenshot of the screen of "Design Studio Settings", under the "Design Studio" tab.