Very slow Incremential Backup with SFTP

[PoleNumCFA]

Hello there,

We are a training center for (mainly) young apprentices based in the south of the France.
We've been using Cobian Reflector for a pretty long time to make backups of our different servers.

We now want to get a "external" backup, on a distant OVH server, using Cobian Reflector.
The amount of data today is ~1.4TB.

The full backup works well, but now we want to do only incremential one.

The thing we don't understand is why the incremential backup takes as long as the full backup, do we have some settings within Cobian Reflector to change so it doesn't take that much time?
It seems that the thing that takes the most of the backup duration is that Cobian is verifying if the file already exist or if it's been modified.

We plan to do a incremential backup / day, but if every backup takes +20 hours, that's not a option.

We use a STFP link with AES-256 crypting to communicate with our distant OVH server (based in France).

We hope you can help us,

Yours sincerely

[cobian]

The first backup of an incremental task will be made full. The second, theird, etc will be really incremental.