Bottleneck just means one part limits the rest. If the CPU is pegged and the GPU is half asleep, more graphics power will not buy much FPS until the processor catches up.
You see this more in competitive multiplayer and busy simulation titles than in pure graphics demos. At 1440p the GPU usually works harder, so CPU limits hurt less.
Do not panic-buy a CPU off a chart. Tell us your games, resolution and current parts. Sometimes the GPU still wins. Sometimes it really is the processor.
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Pick a path, run Find my PC, or message with your games and budget. We will keep it practical.