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Large deflection should usually be set to On.
When set to Off the structural mesh position is not updated during the simulation, so System Coupling loads are applied to the original mesh.
If it is transient consider reducing the time step first for transient cases.
Consider the highest frequency, f, of interest on FEA side, and then dt = 1/20f and Consider what a suitable time-step is for good CFD convergence, or to capture shedding freq. Choose the minimum of the two timesteps.
Also, see if quasi newton stabilization helps. Quasi-Newton Solution Stabilization (ansys.com)
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