Pick a GPU and CPU and see which one holds the other back.
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The Bottleneck Checker shows whether your graphics card or your processor is holding the other back, and by roughly how much, in a typical heavy game.
It is when one part is too slow for the other, so the faster part cannot reach its full potential and sits partly unused.
Not always. Some imbalance is normal, and a gap under about 25% is treated as well balanced here. Larger gaps are worth fixing.