Bottlenecks
The bottleneck is the part of a system which is at capacity. Other parts of the system will be idle waiting for it to perform its task.
In the process of finding and removing bottlenecks, it is important to prove their existence, such as by sampling, before acting to remove them. There is a strong temptation to guess. Guesses are often wrong, and investing only in guesses can itself be a bottleneck.
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