This website measures block and transaction time propagation.. At the time of writing, the time required for 90% of surveyed clients to get a block is about twelve seconds. (I just averaged the last five days.)
How could this be reduced? On a related note, what are the major causes of propagation delay? Is this primarily caused by network latency, network bandwidth, or by clients verifying the block?
- How long does it take for blocks to propagate?
- "Information and Propagation in the Bitcoin network", by Decker and Wattenhofer.