Running the CLI version would make the diagnosis easier:
c:\Users\username\AppData\Local\Duplicacy\duplicacy_win_x64_2.0.9.exe" -d -log backup -stats
The CLI executable can be found under c:\Program Files (x86)\Duplicacy if you installed Duplicacy for all users, or /Applications/Duplicacy.app/Contents/Resources/ if it is a Mac computer.
For a resumed backup, the speed as reported by Duplicacy may far exceed the upload bandwidth at the beginning, because many chunks that already exist on the storage are counted in the calculation. Once it starts to actually upload new chunks the reported speed may drop significantly and become more realistic.