The clock period determined by pfInitClock() on 250MHz IMPACT
  systems is apparently inconsistent with the true clock period.  It
  seems that the actual clock period is that of a 200MHz system.
  Consequently, pfGetTime() will return a time that is .8 (200/250)
  that of the true time.  

  Among other things, this invalidates the calculations done by
  Performer to determine the current video rate.

  As a workaround, you can specify an alternate clock period with the
  PFCLOCKPERIOD environment variable.  If set, PFCLOCKPERIOD
  specifies the clock period, in picoseconds, to be used by
  pfInitClock().  For proper behavior on 250MHz IMPACT systems, use
  40000 for the period.