WordNet (r) 3.0 (2006):

prime time
    n 1: the hours between 7 and 11 p.m. when the largest tv
         audience is available

The Jargon File (version 4.4.7, 29 Dec 2003):

prime time
 n.

    [from TV programming] Normal high-usage hours on a system or network. Back
    in the days of big timesharing machines ?prime time? was when lots of
    people were competing for limited cycles, usually the day shift. Avoidance
    of prime time was traditionally given as a major reason for night mode
    hacking. The term fell into disuse during the early PC era, but has been
    revived to refer to times of day or evening at which the Internet tends to
    be heavily loaded, making Web access slow. The hackish tendency to
    late-night hacking runs has changed not a bit.


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

prime time

   (From TV programming) Normal high-usage hours on a
   time-sharing system; the day shift.  Avoidance of prime time
   was traditionally given as a major reason for night mode
   hacking.  The rise of the personal workstation has rendered
   this term, along with time-sharing itself, almost obsolete.
   The hackish tendency to late-night hacking runs has changed
   not a bit.

   [Jargon File]

   (1995-01-18)