Fedora 23: KDE vs. Xfce vs. GNOME vs. LXDE vs. MATE
First up is the old OpenArena 0.8.5 release, where KDE Plasma 5 is running much slower than the other desktops, perhaps due to its default behavior of not unredirecting full-screen windows.
With the newer OpenArena release, KDE remained the outlier.
When running the open-source Tesseract SDL2 game, KDE Plasma 5 and Xfce were the desktops running slower than GNOME Shell, LXDE, and MATE.