  The decay of a general time dependent structure factors is considered. The
dynamics is that of stochastic field equations of the Langevin type, where the
systematic generalized force is a functional derivative of some classical field
Hamiltonian with respect to the field. Equations of this type are generic and
describe many physical systems. It is usually believed that simple non linear
systems exhibit exponential decay in time, while non linear complex systems
,such as ferromagnets at their critical point,decay slowly as a power law or
stretched exponential. A necessary condition for slow decay is that the
eigenvalues of the appropriate Fokker- Planck operator accumulate at zero for
each momentum q .This is a property of the system. I argue here that when the
necessary condition is obeyed slow or fast (exponential) decay are both
possible for simple linear systems as well as for complex non liner ones. The
actual form of decay is determined by what structure factor we prefer to
observe. An explicit family of linear models in which the "natural" structure
factors decay exponentially is constructed. It is shown how a more complex
structure factor decays in those models with a slow decay form. It is further
shown how in general non linear systems it is always possible to find structure
factors that decay exponentially. The question of actual observation of such
structure factors in experiment and numerical simulations is discussed.
