Back-face culling
Skipping the triangles that turn their back to the camera before they cost anything to draw.
On a closed, opaque model roughly half the triangles are ones you never see anyway — they face away from the viewer and are hidden by the object's front. Throwing them out before rasterization halves the work essentially for free. Which side is the front is decided by winding order: a triangle whose vertices run clockwise (or counter-clockwise, depending on convention) after projection counts as a back face. Alternatively you look at the sign of the dot product between the face normal and the view direction.
This has been built into GPUs since the earliest 3D cards and is controlled by a single render-state flag (glCullFace). But it assumes closed surfaces — leaves, grass and thin flags are two-sided, and there you switch culling off so the back face doesn't disappear.