  The nature of the charge density wave phases in the kagome metal compound
AV$_3$Sb$_5$ has raised many questions and their origin is still under debate.
Here, we combine diffuse scattering (DS) and inelastic x-ray scattering (IXS)
to identify a 3-dimensional (3D) precursor of the charge order with propagation
vector (0.5 0 0.5), which condenses into a 3D-CDW through a first order phase
transition. The quasi-elastic critical scattering indicates that the dominant
contribution to the diffuse precursor is the elastic central peak without
phonon softening. However, the inelastic spectra show a small broadening of the
Einstein-type phonon mode on approaching the phase transition. Our results
point to the situation where the Fermi surface instability at the \textit{L}
point is of order-disorder type with a critical growth of quasi-static domains.
The results go beyond the classical weak-coupling Peierls transition dynamics
and are discussed within models including strong-electron phonon coupling and
non-adiabaticity.
