The beisutsukai are members of a fictional future society that have integrated techniques of rationality so thoroughly that they are capable of performing cognitive tasks currently considered impossible. The members of the society place a high emphasis on speed. For example, the beisutsukai respect Einstein for developing General Relativity Theory, but consider him tremendously slow by their standards for developing the theory over a decade. With the knowledge and techniques of the beisutsukai, a month is considered reasonable time to develop a theory of Quantum Gravity starting from the current state of knowledge.
Possibly derived from "beisu", Japanese pronunciation of Bayes, plus "tsukai", Japanese "user", therefore "Bayes-user".