The thermal time response of polysilicon microfilaments has been characterized at high and room temperatures. We have found cool down time constants that are several times lower at room temperature than for equivalent filaments at high temperatures. It is proposed that the cool down transient can be directly controlled by "substrate quenching," where the polysilicon filament is deflected into direct contact with the substrate. This could result in thermal responses several orders of magnitude faster than conventional structures and may have significant applications for high-speed microbolometer arrays.