We present a model to describe the economic consequences of blockchain protocols on price and liquidity in decentralised exchanges (DEXs). At regular intervals, known as block time, traders bid priority fees to compete for queue position in the next block. Contrary to prevailing assumptions, we show that longer block time and priority fees benefit DEXs. Longer block times improve price efficiency by increasing speculative profits and encouraging traders to acquire information, and they improve liquidity by mitigating adverse selection costs due to increased competition for queue priority.We show that these benefits come at the cost of slower information dissemination, because in equilibrium, traders only reveal their information immediately before the block is created. When participation in informed trading is limited, there is a threshold for block time beyond which markets shutdown.