What is bottleneck?
Bottleneck is a kind of hardware limitation in your computer. A bottleneck occurs when the capacity of an application or a computer system is severely limited by a single component. Components that often bottleneck are graphic card, processor and HDD. Bottlenecks affect microprocessor performance by slowing down the flow of information back and forth from the CPU and the memory. If all of the components of a system are not able to feed the same amount of data at the same speed, a delay is created. For example, a 2GB processor will be severely bottlenecked by an 800MB memory bandwidth.
GPU bottlenecked by CPU
Everyone knows that the most important thing for gaming is GPU. You have HIGH END GPU and you still have low FPS rate? Probably you have to weak CPU for that kind of graphic card.