A computationally highly efficient full-wave model of OCT-scan formation by focused beams in spectral-domain optical coherence tomography (OCT) is presented. Similarly to some previous models, it is based on summation of fields scattered by discrete sub-resolution scatterers and enables accounting for axial and lateral inhomogeneity of the illuminating Gaussian beam. The main feature ensuring high computational efficiency of the described model is that instead of numerical integration of the scattered signal over the receiving aperture we apply analytical description of both the illuminating-beam focusing and collection of the scattered signals over the receiving aperture. Elimination of numerical integration over the receiving aperture has increased the computation speed by a factor of ~ 10^3. This is of key importance for practical feasibility of simulations of 3D OCT data volumes for large amounts (~ 10^5-10^7 ) of scatterers corresponding to realistic densities of cells in biological tissues. We demonstrate the model possibilities by simulating digital refocusing of strongly focused OCT beams in the presence moving scatterers. A novel principle of contrast-agent-free visualization of scatterer flows with velocities typical of blood microcirculation is demonstrated.