This work introduces Reactive Governance, a selective snapshotting framework designed to improve the security, scalability, and gas efficiency of on-chain DAO voting systems. Existing governance models impose trade-offs between security and ecosystem-wide efficiency. Reactive Governance resolves this by applying balance snapshots only when necessary, eliminating continuous checkpointing overhead while preserving resistance to flash loan and late-stake attacks. The paper includes: A formal system model Security analysis Empirical gas benchmarking using Foundry A production-ready Solidity reference implementation Key results demonstrate that governance security can be achieved with minimal marginal cost (~17,715 gas) while eliminating transfer overhead entirely. Full implementation, gas reports, and reproducibility artifacts are available in the associated GitHub repository:https://github.com/timburman/Reactive-Governance