  Due to the increasing interest in blockchain technology for fostering secure,
auditable, decentralized applications, a set of challenges associated with this
technology need to be addressed. In this letter, we focus on the delay
associated with Proof-of-Work (PoW)-based blockchain networks, whereby
participants validate the new information to be appended to a distributed
ledger via consensus to confirm transactions. We propose a novel end-to-end
latency model based on batch-service queuing theory that characterizes timers
and forks for the first time. Furthermore, we derive an estimation of optimum
block size analytically. Endorsed by simulation results, we show that the
optimal block size approximation is a consistent method that leads to
close-to-optimal performance by significantly reducing the overheads associated
with blockchain applications.
