I have 3 TB to sync on three devices. They are preseeded.
Running sha56sum on each device takes around a day.
I preseeded the folders and st is much slower (multiple days) to realize that the folders are already in sync, seems especialy a problem with small files.
Is there a way to accelerate this initial process?
Probably not. Unless you have your Syncthing database stored on a slow medium, that will have somewhat of an impact.
Syncthing calculates hashes for blocks. There is some overhead per file, so lots of small files will take longer than few large ones. It’s all intended to make sure your data is in fact synchronized and safe.