Welcome to the forum @Andrey_SFTDI
Topic title mentions “download speed”. Perhaps you meant “upload speed”?
It could be completely normal, as Duplicati only uploads changed data, and only starts showing speed after the first upload is done. After that, the question is how long it will be before a next remote volume.
How the backup process works shows how uploads happen when a volume is full, thus ready to store.
A simple approach is to watch the home screen to see if the backup is reading through files constantly. Even if a file is read, there might be little to no change, but no reads means no data, and no uploading, meaning the upload speed will slowly decline until another upload happens, because speed is average.
You could also look at your remote share and try to see if a file grows slowly or appears all of a sudden.
You could also watch Duplicati About → Show log → Live → Information to study the uploading timings.
There are several more complex methods, such as using Profiling information, or About → System info. Those are probably better saved for later, if we find out that it’s not just due to having nothing to upload.