CPU and GPU bottlenecks describe different performance limits. A CPU prepares game logic, simulation, draw calls and other work, while the GPU handles graphics rendering. The limiting side depends on the workload.
Signs of a CPU bottleneck
A CPU limit may appear as low or inconsistent GPU utilization while one or more CPU threads are heavily loaded. This is especially relevant in CPU-heavy games, simulation workloads and high-refresh gaming where the system is trying to produce many frames per second.
Signs of a GPU bottleneck
A GPU limit is common when the graphics card is close to full utilization under demanding visual settings. Higher resolution, ray tracing and other graphics effects generally increase the amount of rendering work.
Why 1080p and 4K can behave differently
The same hardware can show different limiting components at different resolutions. Lower resolutions can reduce GPU work and expose CPU limits at high frame rates. Higher resolutions increase pixel-rendering work and can shift the workload toward the GPU.
Do not chase zero bottleneck
There is no single perfect CPU-to-GPU ratio for every application. A balanced system is one that delivers the performance you actually need in the workloads you use.
How to choose an upgrade
Measure the workload first. If the same limitation appears consistently, compare the cost and expected improvement of an upgrade to that component. Also check RAM, cooling, storage and PSU requirements before buying new hardware.