Downloadable Whonix versions come with KDE installed by default. It is possible to uninstall KDE, although doing so is a bit difficult, especially if you want to uninstall all of it and/or to to use a CLI version and/or to install their own desktop environment such as Gnome, LXDE, etc.
Users who build Whonix from source code using Dev/Build Documentation can use optional Terminal-Only build configuration option. And then decide from there.
Whonix with kde runs pretty, anyway I was test it with xfce4 on both of guests, it is notable better on performance , should someone become maintainer , xfce4 is rich on tweaks and apps, and have allmost the 1/3 of kde minimum hardware requirements , i have not time for maintain and perhaps the ability, but i know it is obviusly better for whonix to have xfce’s because of 2 guests run at the same time . I believe xfce is secure as gnome , anyway considered untested and for that unsecure, The other way for see diference on performance is to replace hdd by an sata ssd with at least 120 GB capacity , must install the host there and whonix inside that , this will increase whonix performance for 40% standard *all ready tested.