Dear Ricardo,
What is the rotor speed?
Rules of thumb for selecting the aerodynamic and structural time steps are summarized in the following forum topic: FAST Integrator, step size & precision - #2 by Marshall.Buhl. Granted, these rules are for a flexible turbine operating in turbulence, and it sounds like you are modeling a rigid turbine, presumably in uniform inflow, so, you can probably increase the time step relative to these recommendations.
By the way: If you are truly modeling a a rigid wind turbine, are you only interested in aerodynamic quantities, not including mass/inertial effects. If so, the new driver for AeroDyn under development within OpenFAST may be of interest to you as an alternative to modeling a rigid turbine in OpenFAST with motion prescribed via ExtPtfm: github.com/OpenFAST/openfast/pull/688.
Best regards,