When you bridge assets using Across, you can expect to receive your funds in 1–4 minutes due to the speed of relayers funding users' transfer requests upfront. Relayers wait through the challenge period, taking on 2 hours worth of liveness (instead of the user) before receiving their refund from the liquidity pool.
To understand Across' speed advantage, you need to know what a relayer is. In short, a relayer “relays” information across chains. The relayer is an independent bot, separate from the core contracts. The speed of transactions depends on the speed of relayers fulfilling deposits.
The speed of these relayers is dependent on the relay code, which can be heavily optimized. People are economically incentivized to create exceptionally fast bots, as they want to earn fees paid by users when using across. These fees are set by the user and are a percentage of the deposited amount (to relay from origin to destination chain).
Across is able to do near-instantaneous transfers because we operate optimistically.
In our optimistic approach, the relayer fulfills users' deposits, which can be thought of as providing a loan to the user in exchange for a small fee. The relayer is fronting the money to the user because it trusts UMA’s optimistic oracle. Across differs in this sense as many other chains require multiple sign-offs, blocking the relayer from quickly performing the relay. Nothing is blocking our relayer.
By functioning optimistically relayers relay instantly, and are then refunded after a 2-hour challenge period. During this two-hour challenge period, the user is now out of the picture - they have already received their funds from the relayer. The relayer is trading places with the user in the sense that they’re willing to wait for the challenge window to complete in exchange for a small fee paid by the user.
If something is disputed during the challenge period, the user does not endure any penalties. The user is completely out of the picture and does not accept any risk in this sense when bridging with Across, only the relayer does.