Most often the speed complaints people have with Duplicati are due to the overhead of hashing, encrypting, and compressing the files rather than actual network bandwidth availability.
Once the initial backup is completed runs tend to get quite a bit faster (depending on the amount of file changes you have in each run).
If you want to try and speed things up you can do things like lower the compression level (0 means no compression) or try a different hashing algorithm as discussed here:
Also, you didn’t mention what Duplicati version you’re running but the later Canary versions include improved hashing as mentioned here:
This post has a slightly more detailed list of what Duplicati is doing during the backups: