WordNet (r) 3.0 (2006):

number crunching
    n 1: performing complex and lengthy numerical calculations

The Free On-line Dictionary of Computing (30 December 2018):

number crunching

    Computations of a numerical nature,
   especially those that make extensive use of floating-point
   numbers.  The only thing Fortrash is good for.

   This term is in widespread informal use outside hackerdom and
   even in mainstream slang, but has additional hackish
   connotations: namely, that the computations are mindless and
   involve massive use of brute force.  This is not always
   evil, especially if it involves ray tracing or fractals
   or some other use that makes pretty pictures, especially if
   such pictures can be used as wallpaper.

   See also crunch.

   [Jargon File]

   (1995-03-17)