It was recently shown that phases forming in thin films undergo a coherency state change depending on the film thickness. For Nb-H thin films, the coherency state was reported to change at about 38 nm. In this study the impact of the coherency state on the phase transformation kinetics is investigated for Nb films of two different film thicknesses (25 nm and 80 nm), below and above the state change thickness. The phase transformation in thin metal-hydrogen films can be studied by surface topography analyses via scanning tunneling microscopy (STM) because of the strong local lattice expansion of the hydride precipitates. STM on Nb-H reveals fast phase transformation kinetics for the 25 nm Nb-film, and much slower kinetics for the 80 nm film. This is suggested to be related to the change in the coherency between the Nb-matrix and the hydride precipitates.