Optimistic rollups trust but verify later
Optimistic rollups assume transactions are valid by default and batch them off-chain, posting only compressed data to the main chain. If a challenger disputes a batch, the network replays the computation on-chain to resolve the dispute. This fraud-proof mechanism relies on the existence of at least one honest participant willing to challenge bad state. The tradeoff is a 7-day withdrawal delay to allow time for challenges.
ZK rollups prove correctness upfront
Zero-knowledge rollups generate cryptographic proofs of computation instead. A sequencer batches transactions off-chain, executes them, and generates a proof that the state transition is valid without revealing the transactions themselves. Verifiers check only the proof, which is computationally lightweight. Finality is instant because the proof itself is the guarantee, eliminating the challenge window.
The efficiency and complexity tradeoff
Optimistic rollups require less development effort because verifying computation on-chain is straightforward, making them faster to deploy. ZK rollups demand advanced cryptography and custom hardware for proof generation, increasing complexity but delivering better latency and user experience. Over time, as ZK infrastructure matures, the tradeoff favors the latter.