Slow switching in the threshold region of Mg0.5Mn0.4Zn0.5Fe1. 6O4toroids has been studied to elucidate physical processes with inherently different time constants, confluent in faster switching. Threshold switching from the reset state occurs by a large Barkhausen jump which is considerably delayed from the start of the step field. The delay varies randomly above a minimum value, sometimes exceeding 100 ms. The statistical distribution of this delay has been measured as a function of field and temperature. Interrupted pulse experiments demonstrate that both the delay mechanism and the Barkhausen switching are essentially viscous with a setup time of several μs. The phenomenon is understood in terms of very slow growth of reverse domains by thermal activation, culminating in "avalanche" switchiag of interacting domains.