Delay pools are Squid's answer to rate limiting and traffic shaping. They work by limiting the rate at which Squid returns data for cache misses. Cache hits are sent as quickly as possible, under the assumption that local bandwidth is plentiful.
Delay pools were written by David Luyer while at the University of Western Australia. The feature was designed for a LAN environment in which different groups of users (for example, students, instructors, and staff) are on different subnets. You'll see some evidence of this in the following descriptions.