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 2026 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, profitable MEV opportunities cluster at the end of blocks, limiting the value of priority access. Third, approximately 30% of time-boosted transactions are reverted, indicating that the Timeboost does not effectively mitigate spam. Fourth, secondary markets for reselling express lane rights experience difficulties in sustaining themselves 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 collusion and narrows adoption, highlighting the limitations of auction-based ordering as a mechanism for fair transaction sequencing in rollups.