Xfce

XFCE is a lightweight desktop environment for Linux and BSD. It requires few system resources and therefore runs faster on older/slower computers than do some other desktop environments, such as KDE Plasma. It is not as light as LXDE. XFCE is moderately configurable, but is also a little older in its appearance than other GTK-based desktops, including Gnome and Budgie. However, it is mature, stable and well supported. It can be installed on any Linux distro in addition to whatever desktop is already available by default. However, XFCE is the default desktop environment of Xubuntu and Manjaro, amongst others.

I would recommend you use XFCE if you have an older computer struggling to run KDE Plasma, Budgie, Deepin or Gnome. However, XFCE's cousin, the MATE desktop environment has a similar layout and philosophy whilst being a little bit more configurable.

Used to be great, nowadays sadly the Rat got fat - it's a RAM hog, has bugs in power management (does not override the default xset screen blanking, rendering all its display management options useless) and the wallpaper setting dialog is horrendously slow if you point it at a folder with lots of pictures. Then there's imminent disaster that is GTK4 / libadwaita. When it gets implemented, it will spell an end to all customization... Xfce is one of the many things in Linux that are currently being meticulously ruined. [irony]Thanks devs![/irony]