When designing Zcash 1.0 (“Sapling”) we were concerned about this possibility hitting Zcash: https://github.com/zcash/zcash/issues/95 What we decided to do about it for Zcash 1.0 Sapling was just recalculate the difficulty factor more frequently (https://github.com/zcash/zcash/issues/147), similarly to how Bitcoin Cash has apparently done.