Light baking
Computing a scene's lighting once, offline, and storing the result in textures and probes so it costs almost nothing at run time.
Global illumination — bounced light, soft shadows, colour bleed — is expensive to compute every frame. For static geometry you don't have to: a lightmapper traces all of it once during a build and writes it into lightmaps for surfaces and light probes for moving objects. At run time the GPU just reads the texture.
The trade is flexibility for cost. Baked light cannot change — move a wall or a lamp and the lighting is wrong until you rebake, which can take minutes to hours on a large level. It also demands clean, non-overlapping lightmap UVs. Real-time GI systems (Lumen, DDGI) exist precisely to escape the bake, but baked light still wins on sheer per-frame cheapness.