CPU converts 3D scene, shaders, and lights into final 2D image — takes seconds to hours depending on complexity. No render, no shot.
You're sitting in front of your compositor, having built the 3D scene, all shaders are correct, the lights are set — and now you have to wait. The computer is calculating. Rendering is exactly that: the CPU (or GPU) takes all your 3D information — geometry, textures, materials, light simulation — and calculates a flat 2D image from it, which the viewer will see later. No rendering, no output. Period.
In practice, this is the bottleneck of your workflow. A complex VFX shot with global illumination, ray tracing, and volumetric lighting can easily take 20, 50, or 100 hours per frame — and that's on a farm with 50 cores. That's why VFX studios work with render servers: you write your scene file, upload it to the farm, and hundreds of CPUs calculate in parallel overnight. The next morning, you have your frames. Individual artists usually render at lower resolutions or with reduced ray bounces to get faster feedback — this is called a test render or preview render. The final render then comes in full quality, with all effects, and takes correspondingly longer.
Important: You control the render process via render settings. Sampler count, ray bounces, noise threshold — these parameters influence both image quality and calculation time. An over-configured render with 5000 samples will take three times as long as one with 1000 samples, but may deliver only a marginally better result. This is craftsmanship: depending on the shot and deadline, you optimize this trade-off yourself. Some studios render in passes (diffuse, specular, shadow, matte, ID passes) — this gives you more control in the edit and sometimes saves calculation time, as you can recompose individual passes instead of re-rendering everything.
The engine also plays a role. Arnold, V-Ray, RenderMan — each engine has different algorithms, different speeds, different quality profiles. Arnold is fast and intuitive, RenderMan delivers cinema quality but takes longer. This is not academic — on set or in the studio, you choose the engine based on the deadline and required precision. A render typically runs in the background; you work on other shots or optimize the next one during this time. If something goes wrong — wrong shader, false light — you have to adjust the scene and restart. That's why previews are important.