  Blockchain systems are being used in a wide range of application domains.
They can support trusted transactions in time critical applications. In this
paper, we study how miners should pick up transactions from a transaction pool
so as to minimize the average waiting time per transaction. We derive an
expression for the average transaction waiting time of the proposed mining
scheme and determine the optimum decision rule. Numerical results show that the
average waiting time per transaction can be reduced by about 10% compared to
the traditional no-wait scheme in which miners immediately start the next
mining round using all transactions waiting in the pool.
