Performance, to some extent, depends on the user's needs and expectations. It includes many variables and parameters, and each site may have a different comfort level and different optimums. One system administrator may be satisfied with a 30-second delay during spam filtering, and another may find this unacceptable, but will tolerate a 15-second delay. When processing email, overnight delays of a few minutes may be acceptable, but during the working day, users may discuss emails in telephone conversations and expect them to arrive within seconds. In cases where emails fail to be delivered, or spam filtering does not occur due to system load, then performance is undoubtedly a major problem. There is a trade-off between thoroughness and time to process.
This chapter describes how to identify performance bottlenecks and provides several solutions for different situations.