If MiniB3D actually is slower ( I have no idea if it is ) then it almost certainly *is* because of the rendering, and quite possibly the choice of API.
Sounds totally untrue to me. The code needs to be very, very fast and efficient to sort all textures, surfaces etc, factor in culling and other cpu intensive calculations.
Then finally get down to your game AI and so forth. The CPU can and is stressed by the graphics engine.
Thats a real-world game with levels. If you're going to do 1 room of graphics then sure, the CPU isn't going to be the limiting factor.