Dear George,
For information on how the rotor torque is computed in FAST, please see the forum post found here: Resistant moment of the rotor and of the electric generator.
Regarding your long run times, I’m not sure I understand what you mean when you say, “I use in my simulink model a sample time of 10^-5.” Regardless, have you chosen time steps that are appropriate to your model? There are many good topics on this forum explaining how to choose a proper time step (DT) for FAST. But basically, the highest natural frequency in your model determines the required DT. Algebraic loops in a Simulink model are also known to cause long run times. More information and suggetions on how to resolve a problem regarding algebraic loops are availalable in the Simulink documentation and in the forum post found here: Error Running Test01_SIG.
Best regards,