Very slow local backup to USB disk

I am currently recreating a remote backup from scratch, with the USB disk attached to a local Linux VM.

The backup speed however is very slow with an average speed of 5-6 MB/sec, and I can not relate this to any resource issue. It seems Duplicati is just “taking a break” for a minute in between every volume creation.
During this “break” top reports the VM is 99,2% idle, no I/O wait states, no memory limit, nothing. Neither does the VM host. dd throughput to the same local USB mountpoint is about 60 MB/sec.

I can get a little more speed by increasing the volume size from 100MB to 250MB, average 7-8MB/sec. I have also tried playing around with increasing the blocksize from default 100kb to 1MB, setting the Zip compression level to 1 (instead of default 9), enabling use-block-cache, but these settings don’t seem to make any real difference.

The backup source consists of ± 900 GB, ± 20.000 files. The Linux VM has access to 4 CPU cores, 2GB of memory. Running Duplicati version on Debian 11.

Are these slow speeds to be expected, or might there be something wrong with my setup?

Might be related. On Android I was backing up recently and didn’t format a new sd card. It was backing up at about 5-6MB/sec. I formatted it to Android default (from whatever it was) after seeing some problems (a different issue unrelated issue) and it jumped to about 17MB/sec which is about the fastest I can get Duplicati to backup locally over SSH. That’s using 100MB volume size.

So in short format type seems to have a lot to do with it. The connection type should also have an important part in speed as well eg SSH verses WebDAV, etc. Of course, you also have eg write speed from the read disk, CPU, etc.