  The time domain remains, in many respects, the least explored of parameter
spaces in astronomical studies. The purpose of this article is to encourage the
SPICA community to consider the potential of rapid infrared timing
observations. The specific example considered is that of variable emission from
relativistic jets in compact accreting objects, whose formation and powering
mechanisms we still do not understand. Infrared observations have the potential
to give us fundamental insight on the conditions required for jet formation in
accreting stellar-mass black holes and neutron stars. This is because particle
acceleration is thought to be magnetically-driven, and the spectral transition
between optically-thin and self-absorbed jet synchrotron radiation lies in the
infrared. We review recent observations from WISE showing that we have the
capability to measure key physical parameters of the jet, and their
time-dependence on rapidly-changing conditions in the accretion flow around the
black hole (on timescales of just a few seconds). SPICA will provide a
breakthrough in this field because of its sensitivity and broadband coverage,
and we detail an example SPICA observation on short (tens of milliseconds)
timescales. We believe that SPICA has the potential to make great impact on
time domain science, and we discuss some technical requirements that will
enable this.
