After you click on the application menu (big "R"), Revit reacts slowly, after a long delay (up to 1 min). This problem does not occur with another user logged in.
This delay is caused by Revit searching for files listed in his "file history" and these files are unaccessible. They may be deleted, renamed, or the files are located in a folder or on a server which is currently unavailable for you (e.g. lost connection, removed or renamed network disk mapping).
It can be solved by removing file history from the revit.ini configuration file.
The file revit.ini is located in:
c:\Users\USERNAME\AppData\Roaming\Autodesk\Revit\Autodesk Revit 2015\revit.ini
Open this file with Notepad and remove the file history beginning with the section header:
[Recent File List]
You can also reset Revit by deleting the Revit.ini file. After you re-run Revit, this file will be re-created with default settings and empty file history.