Plex needs to scan the underlying folders in order to find your new files.
Usually subsequent scans won’t take as long as the initial one as it’ll only do a full analysis if changes to a file are detected. Similarly, a sonic analysis is only performed when an album is added to the library (not on every update) – if you have enabled the sonic features.
That’s what it’ll do…
It goes through the directory and checks:
if files/folders have been added
if files/folders have been removed
if media has changed (using e.g. the date added but also some other characteristics to avoid redoing a full analysis if you simply fixed a typo in your file naming).