Ferrofluids can be used to make deformable mirrors having highly interesting characteristics (e.g., extremely large strokes and low costs). Until recently, such mirrors were thought to be restricted to corrections of frequencies lower than $10\mathrm{Hz}$ , thus limiting their usefulness. We present counterintuitive results that demonstrate that the limiting operational frequency can be increased by increasing the viscosity of the ferrofluid. We tested the response of ferrofluids having viscosities as high as $494\mathrm{cP}$ , finding that they could allow an adaptive optics correction frequency as high as $900\mathrm{Hz}$ . We also demonstrate that we can counter the amplitude loss due to the high viscosity by overdriving the actuators. The overdriving technique combines high current, short duration pulses with ordinary driving step functions to deform the mirror. The integration of a FDM in a complete closed-loop adaptive optics system running at about $500\mathrm{Hz}$ thus appears to be a realistic goal in the near future.