Apparently, Quake 3 strictly separates game engine from game content. The latter are packaged as platform-independent bytecode and run in a virtual machine. That is, either interpreted, or, for more common platforms, compiled to machine code. The advantages are, among others, platform independence and sandboxing potentially unsafe game content.
The performance is not bad, considering that we have a double-nested virtualization. The game runs very smoothly on my Chromebook Pixel.