When a project is opened, the contents of the directory tree and the file table are initialized by reading at least the contents of the root directory into memory. Whether the complete project should also be read into memory at project startup or not can be configured in the project settings (Project Settings).
The scanning and refreshing of the project's directories and files is in general performed in the background, so you can immediately start to work after opening a project and you may continue to work while the project is refreshed. If a Refresh is currently in progress, the status bar shows a Refreshing text and symbol.
The initial scanning/refresh is in general much slower than subsequent refreshes due to the system disk cache.