On Blockchain Commit Times: An analysis of how miners choose Bitcoin transactions

Blockchains suffer from a well-known, non-trivial scalability problem: The low throughput (i.e., transactions committed per unit time) of blockchains when paired with the increasingly high volume of issued transactions leads to significant delays in transaction commit times. In a month-long investigation of Bitcoin, we reveal that congestion (i.e., when there exist more transactions than can be included in a block) is typical and that commit times exhibit a significant variance during periods of congestion. Although the feeper-byte dequeuing policy is widely considered the “norm” for prioritizing transactions—and explaining how and when transactions are committed—we show that miners somehow delay a significant fraction of transactions. Such deviations undermine the utility of blockchains for ensuring a “fair” ordering that might be required for some applications.