The Express Lane to Spam and Centralization: An Empirical Analysis of Arbitrum's Timeboost

2025-09-26

AI Summary

Arbitrum’s Timeboost mechanism aims to mitigate MEV in DeFi via auction-based transaction ordering, yet empirical evidence reveals severe deviations from its design goals—exacerbating centralization, failing to curb spam transactions, and undermining fairness. Method: This paper presents the first large-scale on-chain empirical study of Timeboost, analyzing 11.5 million transactions and 151,000 auctions. We combine auction behavior modeling, MEV detection, execution outcome tracing, and incentive structure analysis. Contribution/Results: We find two dominant entities monopolize >90% of auctions; 22% of accelerated transactions are reverted; MEV profits concentrate heavily in block tails; secondary markets are nonfunctional; and DAO auction revenue declines persistently. These outcomes stem fundamentally from endogenous incentive misalignment within Timeboost’s design. Our findings provide critical empirical evidence and concrete design warnings for MEV governance in permissionless blockchains.

Technology Category

• Multiagent Systems: Mechanism Design
• Game Theory and Economic Paradigms: Mechanism Design
• Data Mining & Knowledge Management: Representing, Reasoning, and Using Provenance, Trust

Application Category

• Economics, Online Markets and Human Computation: Trust and reliance of crowd workers and data experts on GenAI
• Web Mining and Content Analysis: Web data provenance, reliability, and authenticity
• Security and Privacy: Data transparency and provenance

Abstract

DeFi applications are vulnerable to MEV, where specialized actors profit by reordering or inserting transactions. To mitigate latency races and internalize MEV revenue, Arbitrum introduced Timeboost, an auction-based transaction sequencing mechanism that grants short-term priority access to an express lane. In this paper we present the first large-scale empirical study of Timeboost, analyzing over 11.5 million express lane transactions and 151 thousand auctions between April and July 2025. Our results reveal five main findings. First, express lane control is highly centralized, with two entities winning more than 90% of auctions. Second, while express lane access provides earlier inclusion, profitable MEV opportunities cluster at the end of blocks, limiting the value of priority access. Third, approximately 22% of time-boosted transactions are reverted, indicating that the Timeboost does not effectively mitigate spam. Fourth, secondary markets for reselling express lane rights have collapsed due to poor execution reliability and unsustainable economics. Finally, auction competition declined over time, leading to steadily reduced revenue for the Arbitrum DAO. Taken together, these findings show that Timeboost fails to deliver on its stated goals of fairness, decentralization, and spam reduction. Instead, it reinforces centralization and narrows adoption, highlighting the limitations of auction-based ordering as a mechanism for fair transaction sequencing in rollups.

Problem

• Analyzes centralization risks in Arbitrum's Timeboost auction mechanism
• Investigates MEV mitigation effectiveness and transaction spam reduction failures
• Evaluates economic sustainability and decentralization impacts of express lanes

Innovation

• Auction-based transaction sequencing mechanism
• Priority access to express lane
• Empirical analysis of Timeboost performance