I just today had to rebuild db with JottaCloud and in my case slowest part of the recreation was getting all the indexfiles. High number of small files and the API latency between requests. Probably nothing much Duplicati can do about that, unless requests are parallelized.
I’m also using Duplicati with OneDrive but indirectly, where I rclone stuff to OneDrive after first updating the backup set(s) locally. I’ve noticed that they’ll easily trigger ratelimits, if trying to transfer small files using high number of parallel connections. Which of course will make the process slower.
Ref: Mono update (JottaCloud TLS issue) -> Database rebuild - Why?