Is this all one backup? How “at first” is it, e.g. is this the initial backup, or an update to existing?
Updates only upload changes, sometimes they take awhile to gather, and speed is an average.
When there is nothing needing upload, average speed sags, but it picks up again on an upload.
You can find a somewhat equivalent (not fully because by default 4 uploads can run at once):
log-file=<path> and log-file-log-level=information will show bytes and times. Divide.
or
log-file=<path> and log-file-log-filter=+*UploadSpeed shows a pre-calculated speed.
You can also get UploadSpeed in a profiling level log, but there’s lots of additional info in them.
Example of an upload:
2025-02-16 09:12:43 -05 - [Information-Duplicati.Library.Main.BasicResults-BackendEvent]: Backend event: Put - Started: duplicati-b65e0789fb07642b19599a2322369f4f8.dblock.zip.aes (49.99 MiB)
...
2025-02-16 09:12:57 -05 - [Profiling-Duplicati.Library.Main.Operation.Backup.BackendUploader-UploadSpeed]: Uploaded 49.99 MiB in 00:00:13.7577721, 3.63 MiB/s
...
2025-02-16 09:12:57 -05 - [Information-Duplicati.Library.Main.BasicResults-BackendEvent]: Backend event: Put - Completed: duplicati-b65e0789fb07642b19599a2322369f4f8.dblock.zip.aes (49.99 MiB)
Although it’s not human-friendly, you can watch uploading status in About → System info.
lastPgEvent has stats like the below, and the calculated speed there should match status:
“BackendFileSize”:20820762,“BackendFileProgress”:6082560,“BackendSpeed”:2019094,
When Progress catches up with size, there’s nothing to do, so speed sags until an upload.
If on the other hand, this is an initial backup, then there are other factors to worry about…