  We suggest a time-effective algorithm to calculate tight focusing of a
collimated continuous-wave laser beam with an arbitrary cross-section light
vector distribution by a high-aperture microscope objective into a planar
microcavity. This algorithm is based on the mathematical fact that any beam can
be decomposed into a superposition -- either finite or infinite -- of
circularly polarized vortex vector beams, which allows one to factorize focal
field into two parts, one of which depends only on distance coordinates $\rho$
and $z$ and the other one only on an azimuth $\varphi$ in cylindrical
coordinates. We compare the suggested algorithm with that based on the direct
use of the double-integral Richards-Wolf method and demonstrate that the former
is at least 5 times faster for single-point computations and at least two
orders faster for typical focal-region computations.
