Gnome-Software is running in the background consuming 200-500mb RAM. If updates are automated or primarily via apt-get, this can be disabled. It autostarts during boot and is hidden from the autostart menu.

vi ~/.config/autostart/gnome-software-service.desktop
# or
sudo vi /etc/xdg/autostart/gnome-software-service.desktop

change NoDisplay=true to false.

Then open Startup Applications via the start menu and disable the Gnome Software entry.