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

killer micro
 n.

    [popularized by Eugene Brooks c.1990] A microprocessor-based machine that
    infringes on mini, mainframe, or supercomputer performance turf. Often
    heard in ?No one will survive the attack of the killer micros!?, the battle
    cry of the downsizers.

    The popularity of the phrase ?attack of the killer micros? is doubtless
    reinforced by the title of the movie Attack Of The Killer Tomatoes (one of
    the canonical examples of so-bad-it's-wonderful among hackers). This has
    even more flavor now that killer micros have gone on the offensive not
    just individually (in workstations) but in hordes (within massively
    parallel computers).

    [2002 update: Eugene Brooks was right. Since this term first entered the
    Jargon File in 1990, the minicomputer has effectively vanished, the 
    mainframe sector is in deep and apparently terminal decline, and even the
    supercomputer business has contracted into a smaller niche. It's networked
    killer micros as far as the eye can see. ?ESR]


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

killer micro

   [Popularised by Eugene Brooks] A microprocessor-based
   machine that infringes on mini, mainframe, or supercomputer
   performance turf.  Often heard in "No one will survive the
   attack of the killer micros!", the battle cry of the
   downsizers.  Used especially of RISC architectures.

   The popularity of the phrase "attack of the killer micros" is
   doubtless reinforced by the movie title "Attack Of The Killer
   Tomatoes" (one of the canonical examples of
   so-bad-it's-wonderful among hackers).  This has even more
   flavour now that killer micros have gone on the offensive not
   just individually (in workstations) but in hordes (within
   massively parallel computers).

   [Jargon File]