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This question already has an answer here:

Why it isn't set to 1 second or smth like? Is there any cause for this decision? Or it's just a random number?

marked as duplicate by Ismael, Badr Bellaj Jul 24 '17 at 8:49

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The original design of 12 seconds (now more due to difficulty adjustment) was first chosen as research suggested that 12.6 seconds is the mean time required to propagate a new block across a vast majority of nodes in a P2P network.

A few concerns with faster block times is - An increase in stale rates, subsequently potential centralisation as a larger pool with high hash rate will be more efficient, creating more blocks which is accepted by a majority of the network. To resolve this, an incentivisation protocol (GHOST) which takes into factor some stale blocks is implemented to determine the main chain backed by the highest PoW (so taking into account "heaviest" instead of "longest" chain), and also providing a block reward to miners.

If you're interested: http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23fc0/P2P2013_041.pdf

Also: https://github.com/ethereum/wiki/wiki/Design-Rationale

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