Although it probably won’t immediately reveal any cause, I think it might be useful to look at throughput over time, preferably as a graph. On Windows, Task Manager shows this nicely for me provided I don’t have any other network-intensive things running. I just look at Wi-Fi and assume the sends are pretty much Duplicati.
Looking at CPU can be useful to see if it looks plausible that Duplicati’s other activities are impeding sends.
Looking at disk can be useful to see if possibly it’s disk-affected.Task Manager only goes to 100% busy, but Resource Monitor can be started from Task Manager for a closer look, such as how overbooked the disk is.
If you weren’t right in the middle of a backup, I’d have suggested Duplicati.CommandLine.BackendTool.exe to try a put of a file without all the other Duplicati processing, just to see if the basic upload method is slow. Because you’re going SO slowly, you possibly could try that, at little risk of slowing down the backup more.
For technical analysis at a network level, netstat can sort of help, and Wireshark definitely can (but is hard).
Getting back to rough estimation, –asynchronous-upload-limit defaulting to 4 says that if you’re truly limited by the network (which sounds quite plausible), you might have 4 files of your 500MB size in your temporary file area (e.g. what echo %TEMP% at a command prompt says). They should be new, and flowing through.