I do this all the time from the web UI.
At the very top of the page, see that icon to the right of the “pause” icon. It looks like a speedometer. Click that.
That brings up this popup window:
From there you can turn on/off/change upload and download limits. One thing to be aware of is this UI measures those in KB/MB/GB per second, NOT mbps or kbps like most network links are measured. The conversion isn’t hard, but I tend to just lean on Google for this so search for “10KB per second in mbps” on Google and you’ll get their interactive calculator and it’ll let you choose whatever units you want to deal with.
A longer term solution would be some sort of traffic shaping function of your router. I’m looking at doing something like this, but haven’t spent any real time on it. Basically I would identify traffic that I think is Duplicati (certain machines on my LAN talking to my backend storage providers) and put it in a low priority queue.
I know my screenshot says I am running a “Canary” build, but this worked for me on the latest public beta too