  Ethereum is one of the most popular platforms for the development of
blockchain-powered applications. These applications are known as Dapps. When
engineering Dapps, developers need to translate requests captured in the
front-end of their application into one or more smart contract transactions.
Developers need to pay for these transactions and, the more they pay (i.e., the
higher the gas price), the faster the transaction is likely to be processed.
Therefore developers need to optimize the balance between cost (transaction
fees) and user experience (transaction processing times). Online services have
been developed to provide transaction issuers (e.g., Dapp developers) with an
estimate of how long transactions will take to be processed given a certain gas
price. These estimation services are crucial in the Ethereum domain and several
popular wallets such as Metamask rely on them. However, their accuracy has not
been empirically investigated so far. In this paper, we quantify the
transaction processing times in Ethereum, investigate the relationship between
processing times and gas prices, and determine the accuracy of
state-of-the-practice estimation services. We find that transactions are
processed in a median of 57s and that 90% of the transactions are processed
within 8m. The higher gas prices result in faster transaction processing times
with diminishing returns. In particular, we observe no practical difference in
processing time between expensive and very expensive transactions. In terms of
accuracy of processing time estimation services, we note that they are
equivalent. However, when stratifying transactions by gas prices, Etherscan's
Gas Tracker is the most accurate estimation service for very cheap and cheap
transaction. EthGasStation's Gas Price API, in turn, is the most accurate
estimation service for regular, expensive, and very expensive transactions.
