I have had problems with saving of files with Impress. The file save process will start but it does not finish. When the saving works, it is usually completed well within a minute. When it does not work, five minutes or more pass and the program becomes unresponsive. I have had this problem only with Impress.
What version of LibreOffice and version of operating system? How big is Impress file (number of slides)? Do you have some special big images? What is file format you are trying to save into=