A bottleneck is a condition in which the limitations in one component prevent the whole system from operating faster. The device with the lowest maximum throughput is the most likely to become a bottleneck if it is in demand. Making any other device faster can never yield more throughput; it can only result in lower utilization of the faster device.
Even if all other components are infinitely fast, a bottleneck holds the system at a stall until it is cleared.
Although a foolproof bottleneck alarm and a direct bottleneck counter aren't available, you can combine several different indicators to look for bottlenecks. The primary indicator is an extended high rate of use on one hardware resource and resulting low rates of use on related components. It is accompanied by sustained queues for one or more services, and slow response time.