Sent dmesg via a private message.
Some details I missed earlier:
- for all these testing I have one UV500 attached to mobo, 2 Barracudas attached to the controller (one not initialized in DSM), 2 external HDDs attached to USB3 ports (one with USB2, one with USB3 interface)
- loader 1.04b DS918+, DSM 6.2.3-25426
What I tried more:
- run Debian 9 live from USB stick - no speed issues
- tried 1.03b for DS3617xs, but apparently my NIC is not supported (machine started, just no network connectivity to it), so got back to 1.04b (1.03b actually did something, I needed to "migrate" disks to DS918+ after changing loader back).
- tried playing around with BIOS settings (enable/disable legacy boot support and likes) - no change
I didn't test network directly, so I think network issues just reflect poor disk performance.