I may need some clarification.
According to the docs I have understood that a rescan will rehash only files that have a modification in their time, size or permission bits.
So if I restart syncthing, why does it takes so long to do a rescan?
Is there something in the configuration that I should check?
As side note I have a 8TB drive splitted in 2/3 TB folders.
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.