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 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.