Guidelines on the best practices
1. If the folder you are trying to export contains lot of content resources i.e. Report exports in a file format, please refrain from exporting the whole folder until and unless you delete the exports. This will reduce the size of the export and doesn't cause a performance bottleneck.
2. You can export import individual reports as well.
3. If the folder is already present in destination server, then the contents are updated with the ones from source environment. If the folder doesn't exist it's created during import.