Texture compression
Block formats like DXT and ASTC that stay compressed in video memory and are decoded by the hardware on each fetch — unlike a PNG, which must be unpacked to raw pixels first.
A PNG or JPEG compresses for disk but has to be expanded to a raw bitmap before the GPU can use it. Texture compression does the opposite: S3TC/DXT (S3 Graphics, 1999), BC1–BC7, ETC, PVRTC and ASTC store the image in fixed 4×4-pixel blocks that the texture unit decodes on the fly. The texture then costs a fraction of the bandwidth and memory even mid-render.
The price is artifacts across steep colour transitions and a fixed, usually lossy ratio. ASTC (ARM, 2012) softened that with variable block sizes. Compressed textures carry their own mipmap levels and pair naturally with texture filtering.