Crystallization kinetics of rapidly quenched metals were theoretically analyzed for the case in which small clusters are formed after the addition of a third element to a binary alloy. It is shown that these clusters act as perturbing particles that affect the kinetics of crystallization due to an effective change in the melting temperature. This reduction in the melting temperature modifies in a sensitive manner the nucleation frequency, and only changes slightly the crystal growth velocity. A small reduction in the melting temperature can change the critical cooling rate for glass formation in orders of magnitude. The melting temperature shift is proportional to the inverse of the size of the perturbing particles.