/FAST
  Sets the following qualifiers that usually improve run-time
  performance:  /ALIGNMENT=(NATURAL, SEQUENCE), /ARCHITECTURE=HOST,
  /ASSUME=NOACCURACY_SENSITIVE, /MATH_LIBRARY=FAST (Alpha only),
  /OPTIMIZE=(TUNE=HOST) (Alpha only).

  The default is /NOFAST.

  If qualifier /STANDARD is specified, /FAST does not set
  /ALIGNMENT=(NATURAL, SEQUENCE).

  Note that programs compiled with HOST architecture and tuning
  optimizations may run slowly on systems with earlier generations of
  Alpha processors than the compiling host system.