  Improving transaction throughput is an important challenge for Bitcoin.
However, shortening the block generation interval or increasing the block size
to improve throughput makes it sharing blocks within the network slower and
increases the number of orphan blocks. Consequently, the security of the
blockchain is sacrificed. To mitigate this, it is necessary to reduce the block
propagation delay. Because of the contribution of new Bitcoin protocols and the
improvements of the Internet, the block propagation delay in the Bitcoin
network has been shortened in recent years. In this study, we identify impacts
of compact block relay---an up-to-date Bitcoin protocol---and Internet
improvement on the block propagation delay and fork rate in the Bitcoin network
from 2015 to 2019. Existing measurement studies could not identify them but our
simulation enables it. The experimental results reveal that compact block relay
contributes to shortening the block propagation delay more than Internet
improvements. The block propagation delay is reduced by 64.5% for the 50th
percentile and 63.7% for the 90th percentile due to Internet improvements, and
by 90.1% for the 50th percentile and by 87.6% for the 90th percentile due to
compact block relay.
