The instability of the paraelectric phase in ferroelectrics, driven by thermal fluctuations, is discussed on the basis of the quantum three-dimensional spin-1/2 transverse-field Ising model (TIM) within the framework of the Green function method. The two-step critical dynamics of the TIM is analyzed through the ferroelectric order-parameter fluctuation spectra observed above the critical temperature Tc. The spectra profiles are given near Tc in explicit form. The slow-down exponents deltac = <img src="http:/img/fbpe/bjp/v30n4/19eq01.gif" align="absmiddle"> and deltas = <img src="http:/img/fbpe/bjp/v30n4/19eq02.gif" align="absmiddle"> are deduced for the slow and fast parts of structural relaxation and are compared with those known from the literature.