In this case cooling speed would be limited by the coolant flow and its thermal capacity and conductivity. You would have to use the cryoprotectant has a coolant. IIUC typical cyroprotectants are not good coolants at that temperature range. Nothing can be a good coolant close to its own glass transition temperature, since by definition their viscosity becomes very high (solid-like) at that temperature.
In this case cooling speed would be limited by the coolant flow and its thermal capacity and conductivity. You would have to use the cryoprotectant has a coolant. IIUC typical cyroprotectants are not good coolants at that temperature range. Nothing can be a good coolant close to its own glass transition temperature, since by definition their viscosity becomes very high (solid-like) at that temperature.