We report that silver nanowire fillers can result in a three-fold decrease in viscoelastic storage modulus of polydimethylsiloxane composites above a low critical filler fraction of ∼0.5%, contrary to theoretical predictions presaging a modulus increase. Similar fractions of silver nanocube fillers result in no such observable effects. Rheology measurements and calorimetric kinetics analyses reveal that high surface area nanowire filler percolation curtails macromolecular mobility via pre-cure gelation, and hinders crosslinking. Our findings on the effect of metal filler aspect ratio on polymer stiffness at low loading fractions would be important for designing nanocomposites for applications.