The Express Lane to Spam and Centralization: An Empirical Analysis of Arbitrum's Timeboost
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 48.5 million express lane transactions and 494 thousand auctions between January 2025 and April 2026. Our results reveal five main findings. First, express lane control is highly centralized, with three entities winning 99.74% of auctions. Second, while express lane access provides earlier inclusion, it is currently mainly used for CEX-DEX arbitrage, limiting the value of priority access for other types of MEV extraction. Third, approximately 30% of timeboosted transactions revert, indicating that Timeboost does not effectively mitigate MEV inefficiencies such as spam. Fourth, secondary markets for reselling express lane rights struggle to sustain themselves due to poor execution reliability and unsustainable economics. Finally, auction competition declined as secondary markets introduced dedicated CEX-DEX execution lanes, enabling pay-per-transaction instead of pay-per-round billing and 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 internalizing long term revenue for the DAO. Instead, it currently allows for collusion and narrows adoption, highlighting the limitations of auction-based ordering as a mechanism for fair transaction sequencing in rollups.