is zero page-thrashing?

OrangePi Zero (512 MB)
Power from 5V 2A supply, connection via Ethernet.
Armbian_5.25_Orangepizero_Ubuntu_xenial_default_3.4.113

I am not strong on CLI so I have installed "lubuntu-desktop" and "gufw" and set them up.
I also have NoMachine's Remote Desktop server installed, to share the Zero's virtual screen with my LAN.

The project is to assess the Zero's ability to run a low volume mail server.
So I have used the iRedMail script to install and set up the components of the mail server.

It seems to be working - I have received the standard opening 4 emails OK.
The problem lies with connecting to the Zero to find out what it's doing.

After a reboot, when there is no user GUI session, and no Remote Desktop connection,
I can sometimes start an SSH connection, and the machine state report says about 50% of memory is being used,
but often it just times out.

I can sometimes start a Remote Desktop connection, but often it just times out.

If i start an RD connection successfully and then start a SSH connection, it reports 83% of memory being used,
and the GUI seems to freeze at the least provocation for at least a minute, with the RD and SSH connections dropping out and in.

I get the impression the Zero is page-thrashing with the memory load of it all, yet
$ sudo swapon --summary
says it is only using 5% of 127 MB.

I need to investigate, but I can't connect to it because it is too busy (I think).

Any suggestions?