The lack of scalability is known to be the foremost obstacle standing in the way of mass adoption of blockchain technology. All existing blockchain projects look for solutions that could improve the performance of their network.

Many emerging projects claim that they have a magic bullet that could solve the problem. However, such assertions are not always valid. Unfortunately, many observers and investors do not realize the core and root of this problem.

Without deep investigation and considerable technical background, it’s difficult to determine hidden bottlenecks and trade-offs. In this post, we are going to discuss one well-known bottleneck that prevents Bitcoin from scaling.

Shortly after the invention of decentralized peer-to-peer network Bitcoin, researchers got interested in what determines the limits of Bitcoin’s scaling. Soon the core issue was determined and described in terms of block propagation time or block propagation delay.

It’s an average time that is needed for the new block to reach the majority of nodes in the network. In a large decentralized network like Bitcoin, whenever the new block is generated, it is broadcasted according to the Gossip protocol. If some node has got the new valid block, it informs nodes connected to it about its new possession.

Then the node transfers this block to those nodes which asked it to do that. Before the block reaches each full-node in the network, it passes through 7 intermediary nodes. It’s important that every honest node verifies the block before relaying it to other peers.

Obviously, the whole thing takes a while. Every new block shakes the network and makes nodes and ethernet connections between them work at full power.

One might argue that since the launch of the network, there have been many improvements to the Gossip protocol. For example, the Bitcoin improvement proposal BIP 0152 introduced the option to transfer only short transaction IDs, instead of the whole list of transactions, in the block body.

However, if the node doesn’t have that transaction in its mempool, it has to ask its peers to transfer it in a separate message. If there is a large number of such transactions in the block, then improvement from BIP 0152 disappears.

Since data transmission is the most time-consuming part of the block relay, researchers got interested in determining how much time is required for a data packet of a certain size to reach 50%, 90%, or 95% of nodes in the network.

It was found that for blocks of a size larger than 20kB, the block propagation delay is nearly proportional to the block size. According to research published in 2013, every extra kB of data in the block caused an extra 80ms of block propagation delay.

Since then, a couple of academic papers and surveys on this topic have been published every year. They update the aforementioned data and discuss various improvement proposals.

Moreover, the site monitors the current state of the Bitcoin network and the block propagation time. Also, it provides charts with historical data on this subject.

The majority of well-established blockchain networks have the same design as Bitcoin. As a result, the block propagation time in these networks obeys the same rules.

Unfortunately, the block propagation time has a massive effect on the blockchain security. The longer the propagation time in the network, the more often miners mine on top of old blocks.

As a result, the forking of the main chain occurs more often, and the percentage of orphan blocks rises. The long propagation delay leads to the so-called Verifiers Dilemma.

Some nodes may find that skipping the block verification step could be a profitable strategy. In this case, they face the risk of mining on top of the wrong block.

However, if block verification time is significant, this strategy could be profitable. Researchers found that long propagation delay reduces the node’s resistance against 51% attacks and selfish mining.

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