Disclosed is a composition comprising, consisting essentially of, consisting of, or produced from polyvinyl chloride, filler, and an impact strength-retaining amount of a modifier wherein the modifier includes is or an ethylene copolymer, an acid anhydride- or acid monoester-modified polyolefin, or combinations thereof. Also disclosed is a process comprising combining an impact strength-retaining amount of a modifier to a blend that comprises or is produced by combining a rigid PVC and one or more fillers. The combining is carried out under a condition sufficient to prevent or minimize the reduction of impact strength of the blend, to reduce the molten viscosity of the blend, or to minimize the loss in stiffness (flexural modulus) of the blend, in comparison to the unmodified blend.