OK, so here are my findings:
I was able to use a previous version of Armbian. Then I went back to the newer version and it worked!
But, there is a difference:
Last time, I did all the connection to NanoPi using a Windows laptop, directly connected to the NanoPi. This tine I was using an older Linux netbook, also directly connected to the NanoPi (ethernet cable).
I am suspecting this has has to do with the ethernet speed, as my older Netbook has a slower 100mbps NIC, and my laptop has a 1000mbps NIC.
I then connected it again to my newer laptop and the sluggish response was back. So I played a bit with the NIC speed, and as I lowered it, the responsiveness was better, but still not perfect.
Lastly, I connected it to my 1000mbps switch, and while it is usuable, it is sluggish to respond: For example, when running ´htop´ on an idle system, the uptime sometime freezes, jumping 2-5 seconds.
As I mentioned before, all this behaviour didn't happen with other images I tried. I suspect that maybe there is a wrong configuration of NIC on Armbian (at least on last version, I haven't had the time to go back again to earlier version and test it again connected to the switch). Maybe it has to do with tx offloading like I read some other board have problems with?
Armbian monitor logs are here:
http://ix.io/1JUw
BTW: The memory chips are NOT from the problematic numbers.
Thanks for your help!