When resolution impacts processor load
At low resolutions like 1080p, the CPU must prepare frame data faster than the GPU can consume it. The processor fills a queue of draw commands that the graphics card works through, making CPU speed the limiting factor. At high resolutions like 4K, the GPU spends more time rendering each frame, giving the CPU breathing room even if it is not the fastest.
The transition point depends on both the CPU and GPU models. A high-end processor paired with a mid-range graphics card may bottleneck at 1080p, while an older CPU with a premium GPU starts to bottleneck immediately.
Shadow quality and ray tracing shift the balance
Graphics settings that stress the GPU, like shadows and ray tracing, push the workload away from the CPU even at lower resolutions. These effects require hundreds of millions of calculations per frame. Texture quality, on the other hand, barely touches the CPU or GPU if VRAM is available, making it a nearly free setting.