Fantom relies on its competitive aBTF consensus engine called Lachesis. Fantom's Lachesis consensus engine achieves fast transaction speeds through its DAG structure, which allows for parallel processing of transactions. This means that transactions can be confirmed almost instantly, rather than waiting for a block to be mined like in traditional blockchain networks. Additionally, Lachesis uses a leader selection algorithm that prevents bottlenecks and ensures high transaction throughput. Overall, these design choices allow Fantom to achieve some of the fastest transaction speeds among blockchain networks.