The debate over whether ZK Rollups or Optimistic Rollups will power the future of Ethereum scaling has been intense. While many herald ZK Rollups as the ultimate solution, real-world experience from operating large-scale Layer 2 networks suggests otherwise. Based on extensive deployment across hundreds of dApps, millions of transactions, and deep integration with EVM-compatible tooling, Optimistic Rollups—not ZK Rollups—are better positioned to deliver scalable, secure, and decentralized Ethereum expansion.
In building Arbitrum, we chose the Optimistic Rollup (OR) model because it aligns best with user needs: security, trustlessness, EVM compatibility, low cost, and full decentralization. Even today, if we were to rebuild from scratch, our choice would remain the same. Let’s explore why.
How Rollups Work: A Foundation for Understanding
Ethereum users submit transactions to deploy or interact with smart contracts. These transactions are more than opaque data—they're requests that trigger specific actions like transferring assets or recording information.
On Ethereum, two critical processes occur:
- Consensus on transaction order: The network agrees on the sequence of transactions.
- Execution and state update: Transactions are executed to compute new blockchain states.
Scaling becomes necessary because executing every transaction on Layer 1 is prohibitively expensive. Enter Rollups, a class of Layer 2 solutions that offload computation while relying on Ethereum for security.
Both Optimistic Rollups and ZK Rollups achieve this by posting transaction data to Layer 1 and using cryptographic mechanisms to prove correctness—without requiring Ethereum to re-execute every transaction.
The Core Difference: Fraud Proofs vs. Validity Proofs
Optimistic Rollups: Trust, But Verify
Optimistic Rollups operate under a simple principle: assume all state updates are valid unless challenged. No cryptographic proof is submitted when a new block is proposed.
Instead, any node can challenge a fraudulent assertion by executing the disputed transaction and generating a fraud proof. A well-designed dispute resolution protocol ensures the honest party wins. Since challenging incorrect claims is incentivized and penalties exist for false assertions, most nodes behave honestly—making fraud rare in practice.
This means OR systems only require nodes to execute transactions during disputes, drastically reducing overhead in normal operation.
ZK Rollups: Prove Everything Upfront
ZK Rollups use zero-knowledge validity proofs—complex cryptographic constructs that mathematically prove the correctness of a batch of transactions. These proofs are verified on-chain by a smart contract.
While verification is cheap, generating these proofs is extremely resource-intensive. Each instruction in a smart contract must be translated into arithmetic circuits and verified via elliptic curve operations—thousands per simple operation like addition.
This creates a fundamental bottleneck: execution cost in ZK systems isn’t just about running code—it’s about proving it cryptographically.
Optimistic vs. ZK: Key Comparison Dimensions
Cost Efficiency Favors Optimistic Rollups
The most significant advantage of Optimistic Rollups lies in operational cost.
- In OR, nodes simply execute EVM code—just like on Ethereum.
- In ZK, each instruction triggers expensive cryptographic computations, creating an inherent cost disadvantage.
Even though only one party generates the proof in ZK systems, large blockchains need many nodes anyway—for querying data, serving users, enabling withdrawals, etc. OR leverages these existing nodes for security at near-zero marginal cost.
ZK Rollups, however, demand specialized hardware or massive parallel computing clusters to generate proofs efficiently—driving up costs and centralization risks.
🔍 Conclusion: Optimistic Rollups have a structural cost advantage due to simpler execution models.
EVM Compatibility: Where Optimistic Rollups Shine
One of our top priorities in building Arbitrum was full EVM compatibility. This means:
- Identical bytecode execution
- Same RPC interfaces
- Seamless deployment of existing Ethereum dApps ("plug-and-play")
Achieving true EVM equivalence is hard. Early attempts reach ~95% compatibility—but fall short in edge cases critical for production use. Only through architectural refinement can full fidelity be achieved.
While some ZK projects claim EVM compatibility, their implementations often lack support for key opcodes like:
ADDMOD,SMOD,MULMODEXP,SELFDESTRUCT,CREATE2- Standard transaction formats and precompiles
Some even plan to drop bitwise operations (XOR, AND, OR) or limit internal contract calls—breaking widely used DeFi protocols.
Moreover, there’s no public benchmark data showing the cost of generating ZK proofs for arbitrary EVM contracts. Given the complexity involved, estimates suggest it would be prohibitively expensive.
🔍 Conclusion: Only Optimistic Rollups currently offer full EVM compatibility at practical costs.
Trustless Visibility: Seeing Is Believing
A core feature of blockchains is trustless visibility—anyone should be able to independently verify the entire chain history without relying on centralized providers.
Optimistic Rollups publish all transaction data on-chain, allowing anyone to reconstruct the full state transition history. This enables:
- Independent node operation
- Transparent auditing
- Reliable event indexing
- Accurate dispute resolution
In contrast, many ZK Rollups prioritize “compression” by omitting historical data. Their validity proofs confirm final balances—but not how those balances were reached.
For example:
Alice sends 1 ETH to Bob → Bob sends 1 ETH to Charlie
But what if the proof only shows:
Alice -1 ETH, Charlie +1 ETH?
Without full data availability, you can't know whether Bob was involved—or if Diana secretly funded Charlie.
Many DeFi applications depend on transaction ordering and interaction history. Hiding this data undermines composability and auditability.
🔍 Conclusion: Optimistic Rollups provide full trustless visibility; many ZK implementations sacrifice transparency for compression.
Fast and Trustless Finality
Users expect timely finality: once a transaction is confirmed, its outcome should be known and irreversible.
Optimistic Rollups separate transaction ordering from execution:
- Sequencers order transactions and post them to Layer 1 every ~60 seconds
- Execution is deterministic → outcome is predictable from order alone
Because data is published frequently and execution is reproducible, users gain fast certainty. Even before a result assertion is posted (every ~1 hour), the outcome is effectively final.
ZK Rollups face a dilemma:
- Publish proofs every minute? → High gas cost (500k–5M gas per proof)
- Publish hourly? → Delays finality; users must trust sequencers during gaps
To maintain fast finality, ZK systems must publish frequent proofs—making them prohibitively expensive—or compromise on decentralization by relying on trusted sequencers.
🔍 Conclusion: Practical ZK Rollups end up mirroring OR designs for timely finality.
Trustless Liveness: Who Can Keep the Chain Moving?
Trustless liveness means any participant can force progress—even if others go offline or act maliciously.
In Optimistic Rollups:
- Any node can submit a state claim by executing transactions
- Requires minimal hardware—just standard compute resources
In ZK Rollups:
- Progress depends on generating validity proofs
- Requires specialized hardware or massive compute farms
- Most users cannot participate directly
If only a few entities can produce proofs, the system becomes de facto centralized. Without open-source tooling for proof generation, there’s no guarantee of liveness.
🔍 Conclusion: Optimistic Rollups enable broader participation and stronger decentralization guarantees.
Bridging: ZK’s Narrow Advantage
ZK Rollups do have one clear benefit: faster withdrawals to Layer 1.
Since validity proofs are instantly verifiable on Ethereum:
- Funds can exit immediately after proof confirmation
- No waiting period required
Optimistic Rollups typically impose a ~7-day challenge window for security—but this delay can be bypassed via third-party fast bridges (e.g., Connext, Hop), which provide instant liquidity at a small fee.
👉 Learn how next-gen rollups are redefining cross-chain asset transfers with instant settlement.
However, this ZK advantage applies only to L2→L1 withdrawals. It doesn’t help with:
- L1→L2 deposits
- Cross-rollup transfers
- Multi-chain interoperability
In reality, users increasingly stay within rich Layer 2 ecosystems like Arbitrum—home to hundreds of DeFi dApps. For multi-chain use cases, the bridging advantage fades.
🔍 Conclusion: ZK’s bridging edge is real but narrow—outweighed by broader usability in OR ecosystems.
Frequently Asked Questions (FAQ)
Q: Are ZK Rollups more secure than Optimistic Rollups?
A: Both inherit Ethereum’s security. ZK uses cryptographic proofs; OR uses economic incentives and fraud proofs. Neither is fundamentally more secure—the trade-offs lie in cost and usability.
Q: Can Optimistic Rollups become faster over time?
A: Yes. Upgrades like Arbitrum Nitro reduce costs and improve data compression. Future innovations in batching and dispute resolution will further enhance performance.
Q: Is EVM compatibility really that important?
A: Absolutely. Millions of lines of audited code, tools, wallets, and developers are built around EVM. Requiring rewrites increases risk and slows adoption.
Q: Will ZK technology eventually surpass Optimistic Rollups?
A: While ZK tech improves, its computational overhead remains fundamentally higher. OR benefits from Moore’s Law-like improvements in general computing; ZK requires breakthroughs in proof generation efficiency.
Q: Do users actually care about trustless visibility?
A: Yes—especially auditors, developers, and institutions. Full transparency enables reliable analytics, compliance checks, and secure integrations across DeFi protocols.
Final Verdict: Optimistic Rollups Lead the Way
After analyzing cost, compatibility, visibility, finality, liveness, and real-world utility:
✅ Optimistic Rollups offer lower costs
✅ Full EVM compatibility
✅ Complete trustless visibility
✅ Fast finality without centralization
✅ Stronger decentralization guarantees
ZK Rollups excel only in one area—fast L1 withdrawals—but even this benefit diminishes with fast bridge services. Meanwhile, their high computational demands limit accessibility and threaten decentralization.
We’re not against ZK tech—it has valuable niche uses. But for general-purpose smart contract scaling, Optimistic Rollups represent the most practical, efficient, and user-aligned path forward.
As we continue advancing Arbitrum with Nitro and beyond, we’re committed to pushing scalability to its theoretical limits—without sacrificing decentralization or compatibility.