  We use ultrafast x-ray diffraction (UXRD) and the polar time-resolved
magneto-optical Kerr effect (tr-MOKE) to study the laser-induced metamagnetic
phase transition in two FeRh films with thicknesses below and above the optical
penetration depth. In the thin film, we identify an intrinsic timescale for the
light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic
material of $8\,\text{ps}$ that is substantially slower than the speed of
sound. For the inhomogeneously excited thicker film, only the optically excited
near-surface part transforms within $8\,\text{ps}$. For strong excitations we
observe an additional slow rise of the FM phase, which we experimentally relate
to a growth of the FM phase into the depth of the layer by comparing the
transient magnetization in front- and backside excitation geometry. In the
lower lying parts of the film, which are only excited via near-equilibrium heat
transport, the FM phase emerges significantly slower than $8\,\text{ps}$ after
heating above the transition temperature.
