How does vacuum tube heating (heater voltage) affect tube performance/sound?
Both the 12AX7 (ECC83) and ECC85 heaters are designed to run on 6.3V, but the ECC85 heaters draw more current because they have to heat up a larger cathode area. This increase in current draw shouldn't significantly affect the heater voltage in a tube amp, since it is only a small proportion of the total transformer loading.   

The Cathodes in the ECC85 have to be larger because this tube has much higher conductance than the 12AX7. It might take a bit longer to heat up due to the increased mass of its cathode assembly. However 20-30 seconds seems a bit too long. 

Is it new? Tubes gradually lose emission as they age. Another possibility is that the circuit was not designed for a tube with such high transconductance, so it takes longer to reach a satisfactory bias voltage.