  Speckle patterns are a powerful tool for high-precision metrology, as they
allow remarkable performance in relatively simple setups. Nonetheless,
researchers in this field follow rather distinct paths due to underappreciated
general principles underlying speckle phenomena. Here, we advise on a universal
metric of intrinsic speckle sensitivity, and on the advantages and
disadvantages of multiple scattering. This will catalyse progress in speckle
metrology but will also translate to other domains of disordered optics which
are undergoing rapid developments at present.
