Which is exactly where the lack of optimisation of Doom 3 source code comes into play. It relies heavily on Intel’s SSE instructions and inlines/intrinsics in the C source code — which was the real point of testing, as narration mentions: to see whether they would work at all, rather then test the performance — and its memory usage is clearly suboptimal. Still, that the game not only simply compiles, but is actually playable — is an achievement in and of itself. 800 MHz Pentium Ms of the era couldn’t run it at any reasonable speeds and even in 640×480 resolution it was basically a slideshow with 2−5 FPS, and even on 2500 MHz Pentium 4s it still ran equally choppily.