/sys$common/syshlp/helplib.hlb FORTRAN, /OPTIMIZE, LEVEL  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Controls the level of optimization performed by the compiler.

  LEVEL=0 is the same as /NOOPTIMIZE and suppresses all optional
  optimizations.

  LEVEL=3 is the default optimization level and provides all optional
  optimizations except memory blocking.

  LEVEL=4 provides all optimizations including memory blocking, which
  can improve performance for non-vectorized applications that
  operate on large arrays.  See the performance guide for more
  details.  LEVEL=4 cannot be specified if /CHECK=BOUNDS is in
  effect.