First, I noticed, that Visual Studio is very slow:
- debugging does not start for hours,
- repository detection, that normally works by using the folder of the currently active file, takes hours,
- the context sensitive menu activated by the right-mouse click does not show language specific functions,
- the save file dialog is unresponsive.
Later, I noticed, that the open and save panel in every application is very slow or unresponsive. When I quit OneDrive, all problems disappeared.
Solution:
Restart OneDrive, or if that does not help, quit OneDrive at least for the time you want to use the open and save dialog.