<p>An efficient algorithm for calculating the spectral-angular dependences of the optical reflectivity, transmittance, and absorption coefficients of a photonic crystal layer that contain narrow resonances, such as bound states in the continuum, has been presented. The traditional approach using the Fourier modal method results in direct calculations on a very fine spectral grid to resolve adequately such resonances, which require significant computational resources. The proposed method significantly accelerates the calculations without loss of accuracy. Its key idea is to divide the resonant layer into two non-resonant sublayers. The scattering matrices for these sublayers are calculated only on a coarse spectral grid. The elements of these matrices are then interpolated onto the required fine grid, and the final optical coefficient is calculated on this grid. It has been shown that the developed algorithm achieves a computational speedup of at least four times compared to direct calculations on the fine grid, while maintaining sufficient accuracy in describing the resonant features. This approach opens up possibilities for the rapid and accurate design of composite photonic structures.</p>