Please read up https://docs.syncthing.net/users/syncing.html#scanning. In short, it does not rehash anything. The only way to force a rehash of everything would to be reset the database.
Scanning can take a lot of time depending on many factors. You’ve only mentioned the size, however what’s more important is the type of storage (HDD vs SDD vs network, etc.) and the number of files. Scanning through millions of tiny files is going to be much slower than just a few large ones. Also, in the case of Windows, the antivirus software may affect scanning time quite heavily.