3D halide perovskites used for low-cost solar cells or light emitters, exhibit a complex lattice dynamics, which is not accounted for in nowadays descriptions of phonons and electron-phonon coupling. We combined several experimental spectroscopy techniques, including inelastic neutron scattering, to investigate low frequency lattice vibrations and relaxations in 3D hybrid halide perovskites. The extensive disorder of the lattice at normal optoelectronic device operation temperatures is similar to pre-melting where only long-range acoustic normal modes survive, while diffraction patterns exhibit extensive diffuse scattering contributions. Until now, electronic structure calculations for cubic halide perovskites are performed by assuming that the potential energy felt by electrons is described with the nuclei clamped at high symmetry lattice sites. This assumption inevitably misses the effect of polymorphism and anharmonicity which are ubiquitous in this class of materials. To address this issue, we developed recently a new computational approach for first-principles lattice dynamics, electron dispersions and electron-lattice coupling in such strongly anharmonic materials at finite temperatures. Acknowledgments The research leading to these results has received funding from the European Union’s Horizon 2020 program, through a FET Open research and innovation action under the grant agreement No 899141. (POLLOC), and under the Marie Sklodowska-Curie grant agreement No. 899546.