The present invention is a highly mineral-filled halogen-free, flame-retardant composition made from or containing a mineral filler, an olefin multi-block interpolymer, and a polar-monomer-based compatibilizer. The invented system has improved elongation at break, achieves a highly flexible, soft compound at high (e.g. > 40 weight percent) filler addition, and achieves and low residual deformation when subjected to the hot pressure test. The invention also includes cables and extruded articles prepared from the composition.