Ubuntu virtual machine runs slow after idle

[PopeFelix]

I'm running Plex on an Ubuntu Linux VM on my FreeNAS system (FreeNAS as the VM host). It seems like if I haven't logged into the VM for a while, it takes a little while longer for me to be able to log in, as if the VM process had been paged out of active memory and into swap. And sometimes the VM is slow to respond after it's been idle for a while. How would I diagnose the cause of this? What can I do to fix it?

CPU: Intel(R) Core(TM) i3-4130 CPU @ 3.40GHz
Memory: 15.8 GiB
Root: SanDisk UltraFit 8 GB USB 3.0 flash drive (<SanDisk Ultra Fit> at usbus0, cfg=0 md=HOST spd=SUPER (5.0Gbps) pwr=ON (224mA))