If you've imported multiple projects or your project has slowed down for whatever reason, it's a good idea to do a little housekeeping:
Save your project.
Go to File>Remove Unused Footage to remove all unused footage from the project.
Go to File>Consolidate All Footage to remove duplicate footage items from the project.
If you only want to keep certain compositions within a project, select them from the project window and choose File>Reduce Project. All unselected compositions and files not used in those compositions will be erased.