For a lightweight but fully-featured desktop, I would recommend Xfce. The more popular KDE and Gnome are bloated, without offering something you can't do with Xfce. MATE and Cinnamon are somewhere in the middle, performance-wise. All things considered, I still think Xfce is the best choice.
For very old machines with (1 Gb of RAM or even less, weak CPU and GPU) I would recommend IceWM as a better choice. Some people might argue IceWM is more like a window manager instead of a "desktop" (perhaps that's why it is not mentioned in the
documentation about desktops). However with some tweaking, IceWM looks great and has all the features of a light desktop. Even though Xfce works well on very old machines, you will get better performance with IceWM - and it uses less memory (~100-150 Mb less than Xfce, depending on your configuration). I have a very old Netbook which runs surprisingly well with FreeBSD and IceWM.
Both Xfce and IceWM are pretty easy to install in FreeBSD. Configuring the desktop to your liking can be done graphically in Xfce, while for IceWM you will need to edit some self-explanatory configuration files (nothing complicated).