The ratio between propagation delay and block interval
As mentioned by Ethan Heilman here:


  According the paper 'Information propagation in the bitcoin network.' by Decker and Wattenhofer. The mean time for a node to see a block is 12.6 seconds, after 40 seconds 95% of the nodes have seen that block.




From my personal experience as a developer, this delay is real and noticeable. Some block explorers catch transactions ~20-30 seconds before others. It may be scary at first but its normal.