VMS Help Sys Parameters, IOTA  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    IOTA specifies the amount of time (in 10-millisecond units) to
    charge to the current residence quantum for each voluntary wait.
    The correct value approximates the cost of a disk I/O neglecting
    wait time.

    This special parameter is used by Compaq and is subject to
    change. Do not change this parameter unless Compaq recommends
    that you do so.