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Optimistic rollup
An optimistic rollup is a layer 2 network that assumes the transactions it posts to Ethereum are valid and does not prove them up front. Instead, there is a window in which anyone can challenge a wrong result with a fraud proof, and an Ethereum contract settles the dispute.
What it means for you. Withdrawing straight back to Ethereum means waiting out the challenge period — roughly seven days — before the funds can be claimed on mainnet. Faster exits exist through liquidity providers who pay out at once and charge a fee for it; check that fee and the wait before moving a large amount.
Source: ethereum.org: Optimistic rollups · checked 4 October 2026
ZK-rollup
A ZK-rollup is a layer 2 network that bundles transactions, executes them off Ethereum, and posts a summary with a cryptographic validity proof. A contract on Ethereum checks the proof and accepts the new balances only if the proof is correct.
What it means for you. Because each batch is proven, withdrawals to Ethereum do not wait through a challenge period; they complete once the proof is verified. The operator that orders transactions can still delay or censor them, and ethereum.org describes an escape hatch: submitting the transaction directly to the rollup contract on mainnet.
Source: ethereum.org: Zero-knowledge rollups · checked 4 October 2026