Cubase saves all your Preferences when it closes which takes awhile. FYI this means if Cubase crashes when closing you might loose any Preferences changes made during that session. But when a Project closes nothing else happens besides closing the file.
While this question is not specific to Cubase 10 or 8, you did post this in the Cubase 8 sub-forum but you reference Cubase 10.5 in the subject line. 