VMS Help FORTRAN, /MATH_LIBRARY, FAST  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Directs the compiler to select the Fast Vector Math version of
  library routines used to implement vectorized references to the
  ATAN, COS, EXP, LOG, LOG10, SIN, SQRT and TAN intrinsics as well as
  the exponentiation operator ** for real data types.  This version
  gives the highest execution performance.  For certain ranges of
  input values, the selected routine may not give a result that is as
  accurate as the version specified by ACCURATE.  Accuracy reduction
  will be at most 2 bits for single-precision routines, and at most 4
  bits for double-precision routines.  This keyword is not negatable.