Probably not much you can do. Depending on if you are doing things like compression and deduplication, there’s a large amount of cpu resources used when doing a backup. Can you please tell us more about the kind of backup you are doing and what size it is, if you are using compression and deduplication, etc.
Additionally, it’s a good idea when you have a slow situation like this to ssh into your NAS and run the command “top” - this will show you what processes are using the most CPU time. And what is even more important is the metric at the top named “CPU Load” - this is basically the number of threads that are in queue to be processed by the CPU. The number is given in something like 1 minute, 5 minute and 15 minute averages. If the 1 minute number in particular is larger than the number of cores for your NAS, then you will absolutely get VERY slow speeds on doing anything.
It’s entirely possible that it’s not just HBS that is the problem, but multiple other processes are taking up thread time and then adding HBS on top of that pushes you over the edge. Additionally, when you log in with the web browser to look at the NAS, that in and of itself tends to use a lot of process time.
Please understand that your NAS can still be slow even through CPU usage percentage is nowhere near 100%. This is simply because of the backlog of threads being processed.
Could you share screenshots of Resource Monitor before the Dropbox sync is running and while the Dropbox sync is running, along with a screenshot of the job list in HBS 3? Thanks!